Diseases

Disease Agents

Plum diseases are caused by different groups of pathogens. The methods of diagnosis and treatment depend on which organism is responsible.

Fungi are the largest group of pathogens. Fungal infections cause brown rot (moniliosis), shot hole disease (clasterosporium), red spot (polystigmosis), rust, powdery mildew, and many other diseases (Seethapathy et al., 2023). Fungi enter plant tissues through natural openings (stomata) or wounds, develop within the tissues, and feed on host cells.

Bacteria are single‑celled microorganisms that cause bacterial canker, bacterial spot, and crown gall. Bacteria usually enter through wounds – pruning cuts, bark cracks, frost damage, or insect injuries (Seethapathy et al., 2023; Westwood, 1993).

Viruses and phytoplasmas are pathogens that live only inside living cells and disrupt normal cell function. The most dangerous plum virus is Plum pox virus (PPV). It can reduce yields by 80–100% and is considered a quarantine pest in many countries (Seethapathy et al., 2023). Phytoplasmas cause witches’ brooms, yellowing, and dieback.

Nematodes are microscopic worms living in the soil. They damage roots, impairing plant nutrition, and often act as vectors for viruses (Westwood, 1993).

Favourable Conditions for Disease Development

Each pathogen has its own preferred conditions, but there are common factors that strongly increase disease risk.

Weather is the main factor. Most fungal infections develop actively in warm, humid weather. Raindrops and dew spread fungal spores, and high humidity helps them germinate (Buckingham, 2010). For example, brown rot is particularly aggressive in rainy springs and summers with frequent rainfall (Agustí, 2010).

Dense planting – hampers canopy ventilation. In the moist, still air inside the canopy, ideal conditions are created for fungi and bacteria.

Weakened trees are more vulnerable. Stress from drought, waterlogging, nutrient deficiency, or frost damage reduces the tree’s natural resistance (Buckingham, 2010).

Improper pruning – leaves open wounds through which pathogens easily enter. Pruning in wet weather is especially dangerous.

Acid or alkaline soil – disrupts mineral nutrition, weakens the tree, and makes it susceptible to infections. The optimum pH for plum is 5.5–6.5 (Mandal et al., 2021).

Pathways of Infection Spread

Diseases spread in different ways, and knowledge of these pathways helps effectively break the infection chain.

Water – rain, irrigation water, and dew are the main carriers of fungal spores and bacteria (Buckingham, 2010). Spores are washed from diseased leaves and twigs onto healthy tissues.

Wind – carries light fungal spores over long distances. Powdery mildew and rust pathogens spread particularly well this way.

Insects – many pests are carriers of diseases. Aphids, for example, transmit viruses (Plum pox virus), and bark beetles introduce fungal spores into the wood (Seethapathy et al., 2023; Westwood, 1993).

Garden tools – secateurs, shears, and saws that have not been disinfected after working with diseased trees carry bacteria and spores to healthy plants (Buckingham, 2010).

Planting material – infected saplings and scions are one of the most dangerous routes for spreading viral and bacterial infections. Always buy planting material from reputable suppliers with certificates (Seethapathy et al., 2023).

Soil – fungal spores (Verticillium, Phytophthora) and nematodes persist in the soil. Therefore, it is not recommended to plant plums where diseased trees of the same species previously grew (Buckingham, 2010).

Key takeaway: most diseases can be prevented. Good agricultural practice, correct site and cultivar selection, and regular orchard monitoring are your first and most reliable defence. In the following chapters, we will cover specific diseases and control measures in detail.

Next chapter: detailed description of fungal diseases of plum – how to recognise them and what to do.

Brown Rot (Monilinia spp.)

Causal agents – fungi of the genus Monilinia (most often M. fructicola, M. laxa and M. fructigena). This is one of the most dangerous and widespread pathogens of stone fruits (Seethapathy et al., 2023; Agustí, 2010).

How to recognise. The disease appears in two main forms:

• Monilial blossom blight (spring form) – affects flowers, ovaries, and young twigs. Flowers turn brown, wilt, and dry up, but often remain hanging on the tree. From infected flowers, the infection moves to young shoots, causing them to wilt and form characteristic burn‑like spots – as if the branch had been scorched by fire. Cracks with gum exudation may appear on the bark.

• Fruit rot (summer form) – brown, rapidly enlarging spots appear on the fruit. Within a few days, the fruit rots completely, becomes soft, and becomes covered with greyish‑white sporulation cushions (often in concentric rings). Affected fruits either drop or mummify and remain hanging on the tree until the next year (Buckingham, 2010; Seethapathy et al., 2023).

Conditions for development. The fungus is active in warm (+15…+25 °C) and humid weather. Prolonged rains during flowering and fruit ripening are especially dangerous. Spores are dispersed by wind, rain, and insects. Infection enters through skin injuries – cracks, insect bites, hailstorm damage (Agustí, 2010).

What to do.

• Prune and burn all affected twigs and mummified fruits – they are sources of infection for the following year. Prune in dry weather, disinfecting tools.
• Collect fallen fruit promptly and destroy it.
• Thin the canopy to ensure good ventilation and rapid drying after rain.
• Spring spraying with Bordeaux mixture (1%) or copper‑containing products before bud break and immediately after flowering reduces primary infection.
• In case of severe disease development during the growing season, apply systemic fungicides (based on tebuconazole, trifloxystrobin, etc.) strictly according to the label, observing pre‑harvest intervals (Seethapathy et al., 2023).

Shot Hole Disease (Clasterosporium)

Causal agent – fungus Wilsonomyces carpophilus (formerly known as Stigmina carpophila or Cladosporium carpophilum). The disease affects all above‑ground parts of the plum: leaves, young shoots, flowers, and fruits (Seethapathy et al., 2023; Agustí, 2010).

How to recognise.

• On leaves – small (2–5 mm) round spots of reddish‑brown or purple colour. After 1–2 weeks, the tissue in the centre of the spot dies and falls out, forming characteristic “shot‑holes” with a reddish border. In severe infections, leaves drop prematurely.
• On shoots and branches – elongated, sunken spots appear, bark cracks, and gum exudes from the cracks. Shoots may die back.
• On fruits – small brownish spots with raised edges appear; spots enlarge, become cracked, and fruits become deformed.

Conditions for development. The fungus overwinters in infected shoots and fallen leaves. In spring, in warm and humid weather, spores are spread by rain and wind. The infection is most active during bud swelling and after flowering (Agustí, 2010).

What to do.

• Sanitary pruning – cut out and burn all branches with spots and bark cracks.
• Collect and destroy fallen leaves and infected fruits.
• Preventive sprays with copper‑based products (Bordeaux mixture, copper oxychloride) are applied before bud break, immediately after flowering, and again after 2–3 weeks.
• During active leaf growth, if the disease appears, apply systemic fungicides (e.g., based on difenoconazole) at 10–14 day intervals (Seethapathy et al., 2023).

Red Spot (Polystigmosis)

Causal agent – sac fungus Polystigma rubrum (Seethapathy et al., 2023; Agustí, 2010). The disease is widespread in regions with wet springs and is often found on old, neglected trees.

How to recognise. In late spring – early summer, bright reddish‑orange, slightly raised spots appear on the upper surface of leaves. The spots enlarge, may merge, covering most of the leaf blade. By autumn, the affected tissues turn brown and leaves drop prematurely. Heavily affected trees lose leaves as early as August, weakening them and reducing winter hardiness (Agustí, 2010).

Conditions for development. The fungus overwinters on fallen leaves as fruiting bodies. In spring, during humid weather, spores are released and infect young leaves. Warm, rainy springs are favourable.

What to do.

• Carefully collect and burn all fallen leaves in autumn – this is the main preventive measure.
• Dig over the root zone in autumn to bury infected leaves in the soil, where spores decompose faster.
• In case of severe annual infestation, apply fungicides (mancozeb, chlorothalonil) at bud break and immediately after flowering (Seethapathy et al., 2023).

Plum Rust

Causal agent – rust fungus Tranzschelia discolor (syn. T. pruni-spinosae). The disease occurs in many plum‑growing regions, especially when wild Prunus species are nearby (Seethapathy et al., 2023; Agustí, 2010).

How to recognise. In mid‑summer, small yellow or pale‑green spots appear on the upper side of leaves. On the lower side of the leaf, beneath these spots, brownish‑red powdery pustules – clusters of fungal spores – form. Severely affected leaves curl, dry up, and fall. Brown sunken spots may appear on fruits. Shoots sometimes show swellings and cracks.

Conditions for development. The fungus needs two hosts: on plum it produces summer and winter spores, while the alternate host is various species of anemone (Anemone). Optimal infection temperature is +18…+24 °C with humidity lasting more than 12 hours (Seethapathy et al., 2023).

What to do.

• Remove wild Prunus and anemone species nearby, if possible.
• Collect and burn fallen leaves in autumn.
• Thin the canopy to improve air circulation.
• At the first signs, treat the foliage with fungicides (based on tebuconazole, propiconazole, mancozeb) at 10–14 day intervals. Treatments are effective at the onset of disease, before mass spore dispersal (Seethapathy et al., 2023; Agustí, 2010).

Other Fungal Diseases

Besides those described above, other fungal infections can occur on plum. Here is a brief list of the most significant.

• Powdery mildew (Podosphaera clandestina, Sphaerotheca pannosa). Appears as a white powdery coating on leaves, shoots, and fruits. Young leaves curl, fruits become spotted and cracked. The disease develops in warm but humid weather. Control: regular pruning of dense areas, treatment with sulphur‑containing preparations or modern fungicides (trifloxystrobin) at the first signs (Seethapathy et al., 2023; Buckingham, 2010).

• Grey mould (Botrytis cinerea). Often affects flowers in cold, wet springs, causing browning and wilting. A fluffy grey coating appears on fruits. Measures: improve ventilation, remove affected flowers and fruits, apply antifungal sprays during flowering (Seethapathy et al., 2023).

• Black knot (Apiosporina morbosa). Forms dark, hard swellings on branches and trunks, which enlarge over time and can girdle shoots. This disease is particularly dangerous in North America. The only reliable method is to cut out and burn infected branches before bud break, disinfecting tools. Fungicides are used as a supplement to pruning (Seethapathy et al., 2023; Buckingham, 2010).

• Sooty mould and flyspeck – a complex of fungi forming dark coatings on fruit skin. They do not penetrate inside but spoil the appearance. Control: canopy thinning, weed removal, and control of insects that secrete sugary exudates (on which sooty mould grows) (Seethapathy et al., 2023).

• Verticillium wilt (Verticillium dahliae and V. albo-atrum) – a soil‑borne fungus that attacks roots and the vascular system. Leaves turn yellow, wilt, and branches die. On cut branches, dark rings are visible in the wood. No cure. Prevention: do not plant plums where nightshades or strawberries previously grew; use resistant rootstocks; remove and burn affected trees (Seethapathy et al., 2023; Buckingham, 2010).

• Phytophthora root rot (Phytophthora spp.) – a fungus‑like organism that attacks the root collar and roots. Symptoms include wilting foliage and dieback of branches. It is especially dangerous on heavy, waterlogged soils. Prevention: ensure drainage, do not overwater the root zone, plant plums on light soils, and use resistant rootstocks (Seethapathy et al., 2023).

General Rules for Fungal Disease Control

1. Prevention is key. A healthy tree, properly planted and cared for, gets sick less often. Apply fertilisers regularly, water during drought, and mulch the root zone.

2. Sanitary pruning and cleaning. Cut out all diseased, dead, and overcrowding branches. Remove and burn fallen leaves and mummified fruits – they harbour overwintering spores.

3. Optimal canopy. The tree should be well ventilated. Remove water sprouts and branches growing inward.

4. Spraying. In most cases, two to three preventive copper‑based applications are sufficient: early spring before bud break, at the pink bud stage, and immediately after flowering. When disease symptoms appear, switch to systemic fungicides, strictly following the dosage and pre‑harvest intervals (Agustí, 2010; Seethapathy et al., 2023).

5. Choose resistant cultivars. Some plum varieties are less susceptible to fungal diseases – check this characteristic when buying saplings.

Remember that regular orchard inspection will help you spot disease at an early stage, when it is easier to stop. In the next chapter, we will cover bacterial and viral infections, which require a different approach to treatment.

Bacterial Diseases

Bacteria enter the plant through wounds – frost cracks, pruning cuts, insect or hail damage. They move through the vessels, causing wilting, necrosis, and the formation of growths. Bacteria can persist for a long time in the tissues of diseased trees and in plant debris.

Viral Diseases

Viruses are sub‑microscopic pathogens that cannot be treated. An infected tree remains a source of infection for life. Therefore, the main strategy is to prevent the virus from entering the orchard and, if detected, to immediately remove the diseased specimen to protect neighbouring trees.

Diseases Caused by Phytoplasmas

Phytoplasmas are bacterium‑like organisms lacking a cell wall and living inside the phloem (the vessels through which photosynthetic products move). They cause systemic growth disorders and often lead to tree death. They are transmitted by leafhoppers (Seethapathy et al., 2023; Westwood, 1993).

General Principles for Controlling Bacterial, Viral, and Phytoplasma Diseases

1. Plant only healthy saplings. Certified planting material is your main defence against many incurable infections.

2. Observe quarantine. If you purchase a sapling from a region with a problematic situation regarding sharka or ESFY, even without symptoms, plant it separately and observe for 2–3 years.

3. Conduct regular inspections. Examine leaves and fruits especially carefully in late June – July, when symptoms of many viruses and phytoplasmas are most apparent.

4. Immediately remove any trees showing signs of viral or phytoplasma diseases. Do not leave diseased trees in the hope that they will “recover” – they will become a source of infection for the entire orchard (Agustí, 2010).

5. Tools – always disinfect secateurs, saws, and shears after working with trees (especially when bacterial diseases are suspected).

6. Vector control – aphids and leafhoppers. Keep the root zone free of weeds, apply insecticides during peak vector flight periods (but this is only a supplementary measure, not a substitute for removing diseased plants).

7. Do not use cuttings from plants that you cannot be certain are healthy.

Remember that bacterial, viral, and phytoplasma diseases are not a death sentence for the whole orchard if you act quickly and correctly. Vigilance and prevention will help keep your plums healthy for many years.

Main Causes of Gummosis

Causes can be infectious (caused by pathogens) and non‑infectious (related to environmental conditions and care errors). Often they act together: frost or mechanical damage to the bark weakens the plant, and through the wound bacteria or fungi enter, intensifying gum exudation.

Diagnosis: Distinguishing Infectious from Non‑infectious

From the appearance of the gum and associated symptoms, one can suspect the cause, but a definitive diagnosis in complex cases requires laboratory analysis. Nevertheless, the grower can use the following indicators.

Signs of infectious origin:

• Around the gum exudation site there are sunken, dying areas of bark (cankers), often with distinct margins.
• On the bark there are cracks, swellings, necrotic spots.
• Gum often exudes from multiple points along the branch, not from a single spot.
• On leaves or fruits, characteristic spots of bacterial or fungal diseases are present (shot‑holing, chlorotic rings, rot) (Buckingham, 2010).
• In bacterial canker, the gum has a specific sour smell.

Signs of non‑infectious origin:

• Gum exudes from a single wound (crack, cut) on the trunk or thick branch, but the surrounding bark appears healthy, without necrosis or ulcers.
• No symptoms on leaves or fruits (no spots, deformities, rot).
• Gum appearance coincides with a frosty winter, hail, or mechanical damage.
• The tree as a whole looks healthy, grows well, and bears fruit.

Important: Even if the initial cause was non‑infectious, bacteria or fungi can enter through the open wound, and then the process becomes mixed. Therefore, any wound with gum should be treated as potentially infected.

Prevention of Gummosis

Preventing gum exudation is much easier than treating its consequences.

1. Correct site and soil selection. Avoid heavy, waterlogged, and saline sites. Plum prefers well‑drained loams with pH 5.5–6.5 (Mandal et al., 2021).

2. Balanced nutrition. Do not over‑fertilise with nitrogen, especially in the second half of summer. Apply phosphorus and potassium fertilisers that strengthen bark tissues and increase winter hardiness.

3. Gentle pruning. Prune only in dry weather, use sharp tools. Disinfect secateurs and saws with alcohol after each tree, and when working with diseased branches – after each cut. Cover large wounds with grafting wax or special pastes.

4. Protection from frost and sun. In autumn, before frosts, whitewash the trunks and bases of scaffold branches with lime plus copper sulphate – this protects against frost cracks and sunburn.

5. Care of the root zone. Avoid standing water, loosen the soil, mulch, but do not place mulch directly against the trunk to avoid bark collar rot.

6. Pest and disease control. Timely treatment of fungal and bacterial infections is the main prevention of infectious gummosis (see chapters 2 and 3).

Eliminating Gummosis: Step‑by‑Step Plan

If gum appears on the tree, act immediately. The procedure is the same for infectious and non‑infectious wounds, but in the first case, treatment is supplemented by spraying the orchard with fungicides or bactericides.

Step 1. Wound cleaning

With a sharp garden knife (preferably disinfected), carefully scrape the gum exudation site and the surrounding bark down to healthy, undamaged wood. Cut away all dried, cracked, and dark areas of bark. The wound should have smooth edges without overgrowth. If gum exudes from a crack along the branch, clean the entire crack.

Step 2. Disinfection

Wash the wound with a 1–3% solution of copper sulphate (10–30 g per 1 litre of water) or Bordeaux mixture. You can also use a 3% solution of iron sulphate. Allow the wound to dry (leave for 1–2 hours in the air).

Step 3. Sealing

After disinfection, apply a protective layer to the wound: grafting wax, paste “RanNet”, a mixture of clay and cow manure (1:1), or oil paint on natural drying oil. The layer should completely cover the wound but not seep under the bark. The seal prevents wood drying, pathogen entry, and promotes healing.

Step 4. Address the underlying cause

• If gummosis is caused by bacterial canker or fungal canker, after cleaning and sealing, treat the whole tree with appropriate products (copper‑based – in autumn and early spring; systemic fungicides – during the growing season) (Seethapathy et al., 2023).
• If the cause is frost crack, apart from sealing, fertilise the tree with potassium‑phosphorus fertilisers and ensure good watering during drought to strengthen it.
• In case of waterlogging – drain excess water (drainage ditches), improve aeration by loosening heavy soils.
• If gum appears at a pruning cut – simply clean and seal the cut; in future, make cleaner cuts without tearing the bark.

Step 5. Monitoring

After treatment, regularly inspect the wound. If gum reappears, repeat cleaning and disinfection. If the canker spreads to new areas, cutting out the entire affected branch may be necessary (Agustí, 2010).

When to Remove the Tree

If gummosis is abundant, the bark is peeling off in large areas, and cankers encircle more than half of the trunk circumference or main scaffold branches, the tree is doomed. It will not be able to restore normal nutrition, will gradually die, and will become a source of infection for neighbours. Such a tree should be uprooted and burned (Seethapathy et al., 2023).

Key point: gum is not a death sentence but an alarm signal. Timely cleaning, disinfection, and addressing the cause can save the tree and extend its productive life. The key is not to ignore the problem and to prevent re‑infection through open wounds.

Sanitary Measures

Sanitation means removing from the orchard all potential sources of infection. Most pathogens overwinter on plant debris, in diseased branches, mummified fruits, and fallen leaves. By removing these “reservoirs”, the infection pressure in the orchard is greatly reduced.

Regular orchard inspection

Inspect all trees at least 2–3 times per season: early spring (before bud break), during flowering, and in mid‑summer. Pay attention to:

• cracks and cankers on bark;
• spots, holes, deformations, and premature yellowing of leaves;
• mummified fruits remaining on branches;
• gummosis and areas of branch dieback.

The sooner you notice a problem, the easier it will be to manage (Agustí, 2010).

Pruning diseased branches

All dry, diseased, damaged, and overcrowding branches should be cut out. Prune in dry weather using sharp, clean tools.

• Branches with signs of monilial blight, cankers, black knot, or dieback should be cut taking 10–15 cm of healthy wood below the affected area (Buckingham, 2010).
• After each cut, disinfect the blade of the secateur or saw – with alcohol, 1% bleach solution, or a medical antiseptic. This prevents the transfer of infection to healthy tissues.
• All cut branches should be immediately burned or removed from the site. Do not leave them in the orchard or put them in compost – spores and bacteria can survive there for years (Seethapathy et al., 2023).

Collection of fallen leaves and fruit

In autumn, be sure to collect and burn all fallen leaves and fruit. Many fungal pathogens (causing red spot, rust, shot hole, brown rot) overwinter precisely on plant debris. In spring, spores are released into the air and infect young leaves and flowers.

• Pay special attention to mummified fruits – they are the main source of brown rot in the next season (Agustí, 2010).
• Leaves can be buried rather than burned, by digging them into the soil to a depth of at least 20–30 cm (when cultivating the root zone), but only if you are sure they are not infected with dangerous diseases. In most cases, burning is safer.

Tool disinfection

A rule that is often ignored, but it is extremely important. Bacterial canker, viruses, and many fungal infections are transmitted through plant sap. If you prune a diseased branch and then a healthy one with the same tool, you transfer the pathogen to the fresh wound.

• Wipe the secateur, saw, and knife with alcohol (70% solution) or a disinfectant after each tree, and when pruning heavily infected branches – after each cut (Buckingham, 2010).
• For sealing large cuts, use only clean paste or grafting wax from a fresh container. Do not dip a dirty tool into the container.

Proper Agronomy

A healthy, vigorous tree resists infections better. Good care reduces stress, strengthens tissues, and lowers the risk of infection. Agronomic practices are your daily work that makes the orchard resilient to diseases.

Site selection and soil preparation

Plum does not like waterlogging, heavy clay soils, or a high water table. Under such conditions, roots suffer from lack of oxygen (asphyxia), the tree weakens, and pathogens of root rots (Phytophthora, Verticillium) easily enter (Seethapathy et al., 2023).

• Choose a site with deep (at least 60–80 cm), well‑drained, loamy or sandy loam soil.
• The optimum soil acidity for plum is pH 5.5–6.5. On very acidic soils (pH < 5), availability of many elements is reduced; on alkaline soils (pH > 7.5), chlorosis often develops (Mandal et al., 2021).
• Before planting, dig over the area, remove perennial weed roots, and add organic matter (well‑rotted manure or compost). Do not plant plum where an old plum stood, nor after raspberries, strawberries, or nightshades (tomatoes, potatoes) to avoid the build‑up of specific pathogens in the soil (Buckingham, 2010).

Watering and mulching

Both drought and excess moisture are harmful.

• Water plums during dry periods, especially during active shoot growth and fruit swelling. Do not allow prolonged drying of the root zone, but also do not overwater (Buckingham, 2010).
• Mulch the root zone with well‑rotted compost, peat, or mown grass. Mulch conserves moisture, suppresses weeds, and serves as an additional nutrient source. However, do not place mulch directly against the trunk – this may cause bark collar rot.
• In rainy periods on heavy soils, make drainage ditches to carry away excess water.

Fertilisation

Balanced nutrition is key to immunity.

• Nitrogen – needed for growth, but excess nitrogen (especially in the second half of summer) causes vigorous growth of young shoots that do not mature well, overwinter poorly, and are easily attacked by bacterial canker and fungal diseases (Agustí, 2010). Apply nitrogen in the first half of the season; reduce it towards the end of summer.
• Phosphorus and potassium – strengthen the root system, increase frost hardiness, and improve tissue maturation. Potassium is especially important for plum: adequate potassium reduces susceptibility to fungal diseases (Agustí, 2010).
• Apply organic matter (well‑rotted manure, compost) in autumn for digging (1–2 buckets per 1 m² of the root zone) or in spring as mulch. Apply mineral fertilisers according to soil analysis, but approximate rates for an adult tree: 80–120 g nitrogen, 50–80 g phosphorus, 120–150 g potassium of active ingredient per year (Agustí, 2010).

Training and pruning

Proper pruning not only removes diseased branches but also forms a canopy that is well ventilated and illuminated.

• Remove branches growing inward, overcrowding, and crossing. In a dense, poorly ventilated canopy, moisture persists longer, favouring fungal development (brown rot, shot hole, powdery mildew).
• Train the tree to a modified central leader or vase‑shaped system (Westwood, 1993). For plum, an open centre (vase shape) is often recommended for better light and air penetration.
• Plum, like other stone fruits, should be pruned not in winter, but in early spring (before sap flow) or in summer (after harvest). Winter pruning of stone fruits increases the risk of wound infection (Buckingham, 2010; Seethapathy et al., 2023).

Protection from frost and sun

Frost cracks and sunburn are common entry points for infection (gummosis, bacterial canker).

• In autumn, before persistent frosts, whitewash the trunks and bases of scaffold branches with lime and copper sulphate (2–3 kg lime + 0.5 kg copper sulphate per 10 L water). Whitewashing reflects sunlight, preventing bark overheating and cracking (Agustí, 2010).
• In regions with severe winters, wrap the trunks with burlap or special material to protect against frost and rodents.

Resistant Cultivars

Using cultivars and rootstocks that are resistant or tolerant to diseases is the most reliable and environmentally friendly protection method (Westwood, 1993). Modern breeding offers cultivars with resistance to sharka, brown rot, bacterial canker, and other diseases.

Resistance to sharka (Plum pox virus)

Sharka is incurable, so resistant cultivars are especially valuable.

• 'Jojo' (Germany) – one of the most well‑known sharka‑resistant cultivars, possessing genetic resistance (hypersensitive response) (Seethapathy et al., 2023).
• In the USA, the transgenic cultivar 'HoneySweet' (formerly Clone C5) has been developed, resistant to sharka (Seethapathy et al., 2023). However, its use may be restricted in some countries; check availability.
• Among traditional European cultivars, relative tolerance to sharka is shown by 'Cacanska Lepotica', 'Stanley', 'Tuleu Gras', 'President' (they may become infected but symptoms are milder and trees maintain productivity longer) (Seethapathy et al., 2023).

Resistance to fungal diseases

• Cultivars with thick, firm fruit skin and an open, well‑ventilated canopy are more resistant to brown rot and shot hole.
• 'Blue Tit', 'Methley', and some Japanese plum hybrids (P. salicina) show good resistance to brown rot in humid climates (Buckingham, 2010).
• 'Stanley' is often recommended as resistant to major fungal diseases in Europe (Agustí, 2010).
• Among American cultivars, 'President' and 'Bradshaw' are resistant to black knot (Apiosporina morbosa) (Seethapathy et al., 2023).

Choice of rootstock

Rootstock affects not only growth vigour and tree size but also resistance to root diseases, nematodes, salinity, and waterlogging.

• For light, dry soils and arid regions – rootstocks Myrobalan B, Myrobalan 29C (Prunus cerasifera), drought‑ and nematode‑tolerant, but susceptible to bacterial canker (Agustí, 2010; Mandal et al., 2021).
• For moist, heavy soils and regions with bacterial canker – rootstocks St. Julien A, GF 655/2 (Prunus insititia), more tolerant to waterlogging and bacterial diseases (Mandal et al., 2021).
• For calcareous (alkaline) soils – rootstocks GF 677 (peach × almond hybrid) and Adafuel, although they are less tolerant to waterlogging (Agustí, 2010).
• Resistance to root‑knot nematodes – Marianna 2624, Myrobalan 29C, Nemaguard (for peach, but sometimes used for plum) (Seethapathy et al., 2023).

When purchasing saplings, ask the seller which diseases the chosen cultivar and rootstock are resistant to in your climatic conditions. Bear in mind that resistance is often relative – in an unfavourable year, even a resistant cultivar may become infected, but the damage will be less severe than in susceptible varieties (Westwood, 1993).

Summary of prevention:

1. Orchard cleanliness – regularly inspect, prune, and burn diseased branches; collect and destroy fallen leaves and mummified fruits.

2. Healthy growing conditions – water properly, fertilise, train the canopy, and protect bark from frost and sunburn.

3. Correct cultivar choice – prefer regionalised, resistant cultivars and rootstocks, especially if dangerous diseases (sharka, bacterial canker) are common in your area.

These three directions, applied together, will create a reliable barrier that prevents diseases from taking over your orchard. Prevention takes time and attention, but it pays off with healthy trees and abundant harvests for many years.

Biological Protection Methods

Biological protection is based on the use of living organisms or their natural compounds to suppress pathogenic microorganisms. These methods are environmentally friendly, safe for humans, bees, and beneficial insects, but often require more careful planning and timely application.

Microbial antagonists – “beneficial” bacteria and fungi

Some microorganisms naturally suppress pathogen development by competing for nutrients and space or by producing antibiotic substances.

• Bacillus subtilis – one of the most studied biological agents. Products based on it (e.g., Fitosporin, FZB 24, QST 713) are effective against powdery mildew, brown rot, grey mould, and some bacterial diseases. The bacteria colonise the leaf and bark surface, preventing fungal spore germination. They are used preventively, before symptoms appear, or at the first signs of disease (Seethapathy et al., 2023).
• Bacillus amyloliquefaciens (strain D747) – similarly suppresses rust, brown rot, and leaf spot pathogens. It is effective both during the growing season and for post‑harvest fruit treatment (Seethapathy et al., 2023).
• Trichoderma – an antagonistic fungus that actively develops in soil and on root surfaces, suppressing root rot pathogens (Phytophthora, Verticillium, Armillaria). Products containing trichoderma are applied to the soil at planting or used to water the root zone at the start of the season (Buckingham, 2010).
• Aureobasidium pullulans (yeast‑like fungus) – effective against fruit brown rot, especially in combination with low doses of chemical fungicides. Applied during fruit ripening (Seethapathy et al., 2023).

How to use bioproducts:

• Apply them preventively, before disease onset, or at the very beginning of infection (at first spots). Bioproducts do not act as quickly or radically as chemicals and cannot stop an already developed epiphytotic (Buckingham, 2010).
• Spray in cloudy but rain‑free weather or in the evening – ultraviolet rays and high temperatures reduce the viability of beneficial microorganisms.
• Repeat treatments every 7–14 days, especially after rain, when the protective layer is washed off.
• Do not mix bioproducts with chemical fungicides in the same tank unless specifically stated – they may be incompatible.

Natural fungicides and immunity stimulants

• Copper sulphate and Bordeaux mixture – traditional, relatively safe copper‑based products. They are permitted in organic horticulture and are effective against many fungal and bacterial diseases (brown rot, shot hole, bacterial canker) when applied before flowering and after leaf fall (Agustí, 2010).
• Sulphur (colloidal, micronized) – effective against powdery mildew and rust. Works as a contact fungicide; spray foliage at the first signs of powdery mildew, but not in hot weather (above +28 °C) to avoid leaf burn.
• Mineral oils – applied during dormancy (before bud break) to suppress overwintering fungal spores and also to control some pests. Effective against scab and sooty mould.
• Plant extracts – infusions of garlic, wormwood, horsetail – can have a suppressive effect at the initial stages of disease, but their efficacy is much lower than that of specialised products. Use them as an auxiliary measure or on small areas.

Remember: biological methods do not give 100% protection in years with high infection pressure. They work well in healthy orchards, but in neglected or very humid conditions they often need to be supplemented with chemicals (Buckingham, 2010).

Chemical Protection Methods

Chemical fungicides and bactericides are the most reliable means during disease outbreaks. They act quickly and effectively suppress pathogens. However, their use requires caution: observe dosages, application timings, and especially pre‑harvest intervals (the time from the last treatment to harvest).

Classification of fungicides

By mode of action, products are divided into two broad groups.

Contact (protective) – form a protective film on the plant surface and kill pathogen spores upon contact. They do not penetrate tissues.

• Advantages: fast‑acting, cheaper, lower risk of resistance.
• Disadvantages: washed off by rain, require frequent reapplication, do not cure established infections within tissues.
• Examples: Bordeaux mixture, copper oxychloride, sulphur, mancozeb (products containing these substances).

Systemic – penetrate plant tissues, move through the vessels, and suppress the pathogen from within. They not only protect but also cure plants at early infection stages.

• Advantages: not washed off by rain, long‑acting, can stop disease after infection.
• Disadvantages: more expensive, pathogens may develop resistance with frequent use (resistance), so they should be alternated with contact products.
• Examples: tebuconazole, difenoconazole, trifloxystrobin, fludioxonil, pyraclostrobin (active substances in popular fungicides).

Main products for plum protection (examples)

Product names may differ between countries; focus on the active substance.

Rules for safe use of chemicals

• Read the label – dosages and pre‑harvest intervals depend on the specific product and crop. Never exceed the dose.
• Observe pre‑harvest intervals – the period during which fruits cannot be harvested after treatment. For systemic products it is usually 14–21 days, for contact – 7–10 days.
• Alternate products – do not use the same active substance more than 2–3 times per season to avoid resistance. Alternate systemic and contact fungicides, as well as different active substances (e.g., tebuconazole → difenoconazole → mancozeb → copper) (Agustí, 2010; Seethapathy et al., 2023).
• Spray in calm weather – early morning or late evening, when bees are not active. Avoid treatments on hot days (above +25 °C) and before rain – efficacy is reduced.
• Use protective equipment – gloves, goggles, respirator, protective suit. Many fungicides are toxic to humans upon skin or mucous membrane contact.
• Do not exceed the recommended application rate – this does not increase efficacy but may cause phytotoxicity (leaf and fruit burn) and residue accumulation in the crop.
• Stop treatments 3–4 weeks before harvest, even if the pre‑harvest interval is shorter – this reduces the risk of chemical residues in food.

Treatment schedule by growth stages (calendar)

Integrated Pest Management (IPM) for the Orchard

Integrated protection is a systematic approach that combines all available methods: agronomic, biological, mechanical, and chemical. Its goal is not to eliminate all pathogens (which is impossible) but to keep their populations below the economically damaging level – i.e., at a level where they do not cause significant loss of yield or tree health (Westwood, 1993).

Principles of integrated protection

1. Prioritise prevention. A healthy orchard, proper agronomy, resistant cultivars, and sanitation are the foundation. This reduces the need for chemical treatments.

2. Regular monitoring. Inspect trees at least weekly during active growth. Detect disease at the earliest stages. Keep a diary of observations, record weather conditions and disease outbreaks – this will help predict risks for the following year.

3. Economic injury level. Not every disease requires immediate chemical application. For example, a few spots on old leaves in late summer do not threaten the harvest. Treatment is justified when the disease begins to spread to young leaves, fruits, or shoots, or when humid weather favourable for an epiphytotic is forecast (Buckingham, 2010).

4. Use chemicals as a last resort. Chemical products are the “last line of defence”. They always follow agronomic and biological measures, not replace them. Apply them only when other methods have failed, or when weather conditions create a high risk of mass infestation (Agustí, 2010).

5. Alternate methods. Throughout the season, alternate bioproducts, contact and systemic fungicides. This reduces the load on the orchard ecosystem and prevents pathogen resistance to a single substance.

Example of an integrated protection plan for the season

• Autumn: leaf and fruit collection, sanitary pruning, whitewashing of trunks, copper‑based treatment after leaf fall.
• Winter: check bark for cracks and rodents; if necessary, clean and seal wounds (in mild weather).
• Early spring (before sap flow): pruning, copper‑based treatment on dormant buds.
• Pink bud stage: in wet weather – apply a bioproduct (Bacillus subtilis) or, if the risk of brown rot is high, a systemic fungicide.
• After flowering: apply bioproducts or contact fungicides (as needed). Remove and burn branches affected by brown rot.
• Summer: regular inspections. If leaf spots or powdery mildew appear, apply bioproducts or specialised fungicides with alternation. Remove fruits showing rot.
• Late summer – early autumn: maintenance treatments as needed. Stop all chemical treatments 3–4 weeks before harvest. Collect fallen fruit.

Combined use of biological and chemical agents

Bioproducts complement chemical fungicides well. They can be used in the following scheme:

• Early season (before flowering) – chemical treatment (copper or systemic) to sharply reduce the infection pressure.
• Growing season (after flowering and through summer) – alternate bioproducts (every 10–14 days) and, if necessary, chemical treatments at intervals of at least 2–3 weeks.
• Scenario: after rain, a brown rot outbreak is forecast – first apply a bioproduct (if the disease has not developed); if symptoms appear, apply a systemic fungicide to stop the infection, then revert to bioproducts to maintain protection (Seethapathy et al., 2023).

Important: do not mix bioproducts and chemical fungicides in the same tank without compatibility testing. Usually apply them separately, with an interval of 3–5 days, so that the beneficial microorganisms have time to colonise the surface before contact with chemicals.

Summary of Plant Protection

Protecting plums from diseases is not a one‑off activity but a systematic effort throughout the season. Key success rules:

1. Start with prevention – a healthy orchard gets sick less often.

2. Regularly inspect trees – disease is easier to stop at the very beginning.

3. Use biological methods as the basis of protection – they are safe and maintain ecological balance.

4. Use chemicals only when necessary – as “emergency aid” during disease outbreaks, strictly following dosages and pre‑harvest intervals.

5. Combine methods within an integrated approach – no single method works perfectly alone.

Remember that the main goal is not just to cure a sick tree, but to create conditions in which diseases will not re‑occur. A systematic approach, patience, and regular care are the three keys to success in protecting your orchard.