Physiological disorders

Symptoms

• Stunted growth, yellowing of lower leaves
• Leaves become small, with chlorotic spots
• Plants stay in place, do not produce side shoots
• With severe cooling – wilting even when the soil is moist
• Roots become brownish, thin, and poorly branched

Causes

Zucchini roots work poorly when soil temperature drops below 15 °C. At 10–12 °C, water and nutrient uptake decline sharply. Phosphorus is particularly affected – it becomes unavailable in cold soil. This phenomenon is called chilling‑induced root inhibition (Tachibana, 1987; Wien and Stützel, 2020).

Consequences

The above‑ground part receives insufficient nutrition. Leaves turn yellow (especially old ones), growth slows. Flowering is delayed, and early fruit sets may drop. The plant becomes more susceptible to root rots, as weakened roots are more easily attacked by fungi.

What to Do

• Do not rush planting. Wait until the soil at 10 cm depth warms to 12–14 °C. For early planting, use raised beds (20–30 cm high) – they warm up faster (Swiader, 1992).
• Use dark mulching materials. Black film or agrofabric raises soil temperature by 2–4 °C, which is critical in cool springs.
• Grow seedlings in pots. Seedlings with a well‑developed root system tolerate cool soil better than direct sowing.
• If a cold snap comes suddenly, set up a temporary cover of hoops and agrofabric. This retains heat both in the soil and in the ground‑level air.
• Water only with warm water (not below 18 °C). Cold water further cools the root zone and worsens the stress.
• During prolonged cold spells, you can apply a foliar spray of monopotassium phosphate (10–15 g per 10 L water) – phosphorus in an available form helps roots "wake up".

Symptoms

• Leaves and shoot tips become watery, dark green, as if scalded
• After a few hours, tissues turn brown, dry out, and become papery
• Young fruit sets and flowers die (turn brown and fall off)
• With severe frost (below –2 °C), the entire above‑ground part dies

Causes

Frost causes ice crystals to form in the intercellular spaces. They rupture cell walls, and after thawing, the cells simply "collapse" – hence the watery appearance. Zucchini is extremely sensitive to negative temperatures, especially during flowering and fruiting (Rana, 2018).

What to Do

• Watch the weather forecast. On nights with frost risk, cover the plants with non‑woven fabric (spunbond, lutrasil). This provides protection down to –3…–5 °C.
• Use sprinkler irrigation (if you have an irrigation system): turn on a fine mist 2–3 hours before the expected frost. When water freezes, it releases heat, protecting the plants. This method requires precise calculation, but it is effective on large areas (Mondal et al., 2020).
• If frost has already occurred and leaves are damaged, do not prune them immediately. Give the plant 1–2 days – sometimes damaged tissues dry out, and healthy buds produce new shoots. Remove completely dead leaves later.
• For quick recovery, spray the plants with a solution of Epin (or another anti‑stress agent) and give a root drench with a complex fertiliser high in phosphorus and potassium.
• In regions with frequent frosts, practise growing zucchini in raised beds with covers or in plastic tunnels.

Symptoms

• Leaves become pale, edges dry out and curl upward
• On fruits appear sunken light‑brown or whitish spots (sunscald)
• Flowers and small fruit sets drop before developing
• Plants look depressed, growth slows
• In severe heat (>35 °C), leaves curl completely into tubes to reduce evaporation

Causes

At temperatures above 30–32 °C, photosynthesis is disrupted in zucchini. Respiration becomes more intense, and the plant spends more energy than it produces. At 35–40 °C, pollen becomes sterile, and flowers are not pollinated. This is one of the main reasons for lack of fruit set in midsummer (Wehner et al., 2020; Wien and Stützel, 2020).

In addition, overheating of the soil (above 30 °C) inhibits roots. They stop absorbing water, and the plant suffers from water deficit even when watered.

What to Do

• Choose varieties with good heat tolerance. For example, hybrids with a powerful leaf rosette that shades the fruits. Varieties with dark green fruits suffer more from sunburn than light‑green or yellow ones (Narina, 2016; Wehner et al., 2020).
• Ensure regular watering – in hot weather, zucchinis need moisture daily, but without waterlogging. It is better to water early in the morning or in the evening when the sun is not intense.
• Mulch the soil with straw, mown grass, or light‑coloured agrofabric. This not only retains moisture but also protects roots from overheating.
• Shade the plants during the hottest hours. You can stretch shading netting or use tall neighbouring plants (corn, sunflower) to provide light shade.
• Spray leaves with water in the morning to lower temperature and increase humidity. However, avoid getting water on exposed fruits – droplets can act as lenses and cause burns.

Symptoms

• Plants look "shocked": leaves droop even if the soil is moist
• Young leaves curl and become crinkled
• Flower and fruit set drop (especially on plants that have just started fruiting)
• Growth slows for several days

Causes

Zucchini becomes accustomed to certain conditions. When a cool night (10–12 °C) is suddenly followed by a hot day (above 30 °C), the plant cannot quickly readjust its metabolism. This causes a temporary hormonal imbalance and osmotic shock (Mondal et al., 2020; Pessarakli, 2016).

What to Do

• Mitigate fluctuations with covers during cool nights (even if it is hot during the day). Remove the cover only after the air has warmed up.
• Maintain stable soil moisture. Moist soil heats and cools more slowly, smoothing out daily variations.
• Use drip irrigation – it does not cool the soil abruptly, unlike sprinklers.
• During periods of sharp fluctuations, do not apply aggressive fertilisers (especially nitrogen) and do not prune leaves – this adds extra stress.
• When fluctuations are forecast, 1–2 days beforehand, give a foliar spray with potassium humate – this increases plant stress resistance.

Symptoms

• Leaves lose turgor, droop and become wilted even in the morning hours
• Leaf edges dry out, become brittle, and turn brown
• Plant growth slows, internodes shorten
• Flowers and young fruit sets drop in large numbers
• Fruits become small, with rough skin, and may have a bitter taste
• With prolonged drought, leaves yellow from the bottom up, old ones die

Physiological Causes

When moisture is lacking, the plant closes its stomata to reduce evaporation. This simultaneously stops the intake of carbon dioxide for photosynthesis. Productivity drops. In addition, the transport of nutrients – especially calcium, which moves only with water flow – is disrupted. This is one of the main causes of blossom‑end rot in fruits (Mondal et al., 2020; Swiader, 1992).

Drought also shifts the hormonal balance toward stress hormones (abscisic acid), which inhibits flowering and stimulates fruit set drop. The plant "saves" resources, sacrificing reproductive organs for survival (Wien and Stützel, 2020).

Moreover, under dry conditions, zucchinis accumulate cucurbitacins – bitter compounds that make fruits inedible. This is a protective response to stress (Wehner et al., 2020).

What to Do

• Water regularly and deeply. Zucchinis need not frequent shallow watering, but rare, heavy watering – so that water penetrates to a depth of 30–40 cm, reaching the entire root system. Guideline: 10–20 litres per plant 2–3 times a week in hot weather.
• Water in the morning or evening so that water does not evaporate instantly and does not burn leaves.
• Mulch the soil with a 5–10 cm layer (straw, mown grass, compost). Mulch retains moisture, reducing evaporation by 30–50 %, and protects roots from overheating.
• Loosen the soil after watering, as soon as it dries slightly. This breaks the soil crust and improves air access to roots.
• Monitor the weather – on hot and windy days, water demand increases 1.5–2 times.
• Drip irrigation is the best choice for zucchinis. It delivers water directly to the root zone without wetting leaves and without provoking fungal diseases.

Symptoms

• Leaves turn yellow (especially lower ones), growth slows
• Plants look wilted and depressed despite wet soil
• Leaves may curl or develop spots
• Roots become brown, slimy, with a putrid smell
• Plants are easily pulled out of the soil – the root system is destroyed
• Fruits are often affected by rots (especially those lying on wet ground)

Physiological Causes

Zucchini roots, like all roots, need oxygen for respiration. In water that has filled all soil pores, there is almost no oxygen. Roots switch to anaerobic respiration (without oxygen), which produces toxic substances – ethyl alcohol, lactic acid, aldehydes. They poison and kill root cells (Mondal et al., 2020).

Without healthy roots, the plant cannot absorb water and nutrients – even if there is plenty of water in the soil. A condition resembling drought occurs: physiological wilting. Weakened roots become easy prey for fungi Pythium, Phytophthora, and Fusarium, leading to root rots (Kemble, 2022).

Waterlogging is one of the main causes of damping‑off of seedlings and death of young zucchini plants.

What to Do

• Choose sites with good drainage. Zucchinis do not tolerate heavy clay soils where water stagnates after rain.
• Form raised beds (20–30 cm) – they ensure runoff and warm up better.
• Do not over‑water. Rely on soil condition: the top layer should have time to dry out between waterings.
• Loosen the soil to improve aeration. If the soil is compacted, make punctures with a fork in the rows.
• After heavy rains, lift the fruits onto supports (boards, straw) so they do not lie on wet ground and rot.
• If waterlogging becomes chronic (low spot, flooding), consider installing drainage ditches to divert excess water.
• To help roots recover after waterlogging, apply a foliar spray of monopotassium phosphate (15 g per 10 L water) and spray with Epin or Zircon to stimulate new root growth.

Symptoms

• Fruits crack – longitudinal or transverse cracks on the skin
• Plants drop fruit sets and young fruits
• Leaves curl, growth slows
• Fruits become irregularly shaped (pear‑shaped, curved)
• Fruit colour may be uneven – blotchy or pale

Physiological Causes

Cells of zucchini fruits grow very quickly. If, after a dry period, you give abundant watering, the cells begin to rapidly absorb water and expand. The skin (exocarp) has already lost elasticity during the dry spell – it cannot stretch in time. The result – skin rupture, i.e., cracks (Mondal et al., 2020; Swiader, 1992).

In addition, a sudden change in water regime disrupts the transport of calcium to young tissues. This can trigger blossom‑end rot even when there is enough calcium in the soil – the plant simply cannot deliver it in time (Mondal et al., 2020).

Uneven watering also increases stress, which stimulates the accumulation of cucurbitacins and worsens fruit taste (Wehner et al., 2020).

What to Do

• Water regularly, avoiding strong fluctuations in moisture. Better 2–3 waterings per week of 10–15 litres each than "flood‑then‑drought".
• Use drip irrigation – it provides the most even wetting of the root zone without sharp fluctuations.
• Mulching helps smooth out moisture variations: mulch slows evaporation and keeps moisture in the soil longer.
• In hot weather, increase watering frequency, but not the volume per dose – this reduces the risk of waterlogging and ensures stable moisture.
• If using sprinklers, check the uniformity of water distribution across the plot. Often the edges of beds receive less moisture than the middle.
• During active fruiting, do not allow the soil to dry deeper than 5–10 cm. Check moisture with your finger at 10 cm depth: if the soil is dry – it is time to water.

Nitrogen (N)

Role: component of proteins, chlorophyll, nucleic acids. Ensures growth of green mass.

Symptoms (Mondal et al., 2020; Kemble, 2022):

• General yellowing of leaves, starting with the lower, older ones
• Growth slows, stems are thin, leaves small
• Plant looks weak, bush does not spread
• Fruits are pale, small, their number reduced

Causes: poor soils, leaching of nitrates by rain, insufficient organic or mineral fertiliser application.

What to Do:

• Apply organic matter (well‑rotted manure, compost) at planting and as mulch.
• Give a root drench with urea or ammonium nitrate (20–30 g per 10 L water per 1 m²) at the start of growth.
• Do not overdo it: excess nitrogen during fruiting leads to lush growth and delayed fruit set – see below.
• Use a foliar spray of carbamide (1–2 %) for a quick effect in acute deficiency.

Phosphorus (P)

Role: energy metabolism (ATP), synthesis of nucleic acids, root development, flowering, and fruit ripening. Phosphorus moves slowly in the plant, so deficiency affects young organs.

Symptoms (Mondal et al., 2020):

• Leaves take on a bluish‑green or purple tint, especially on the underside
• Growth stunted, plants dwarfed, internodes short
• Flowering weak, fruits set poorly
• Roots are underdeveloped

Causes: phosphorus is immobile in soil and poorly absorbed at low temperatures, high acidity (pH < 5.5), or excess iron and aluminium.

What to Do:

• Apply phosphorus well in advance (in autumn or spring under digging) – it does not leach but must be in the root zone.
• Use water‑soluble forms (monopotassium phosphate, diammonium phosphate) for quick feeding.
• In cold soil – foliar spray with monopotassium phosphate (10–15 g per 10 L) to bypass weak root uptake.
• Maintain soil pH at 6.0–6.5 – phosphorus is most available in this range.

Potassium (K)

Role: regulates water balance (stomatal opening), carbohydrate transport, protein synthesis, stress resistance. Potassium is especially important during fruiting – it improves fruit size, colour, and taste.

Symptoms (Mondal et al., 2020; Kemble, 2022):

• Starts from edges of old leaves: they turn yellow, brown, and die ("marginal scorch")
• Leaves curl downward, become wrinkled
• Fruits small, deformed, with spots and poor taste
• Plants become more susceptible to diseases and drought
• At later stages – leaf necrosis

Causes: sandy soils (potassium leaches easily), lack of potassium fertilisers, excess nitrogen and calcium that compete with potassium.

What to Do:

• Apply potassium fertilisers (potassium sulfate, potassium magnesium sulfate) as a base and in top dressings. Potassium chloride is undesirable for zucchinis – they are sensitive to chlorine.
• During fruiting – regular potassium top dressings every 7–10 days (20 g potassium sulfate per 10 L water per 1–2 m²).
• Can combine potassium with irrigation (drip).
• In acute deficiency – foliar spray with potassium sulfate (0.5–1 % solution).
• Remember: potassium and magnesium are often deficient together on light soils – potassium magnesium sulfate can help.

Magnesium (Mg)

Role: central element of chlorophyll, participates in photosynthesis and enzyme activation. Magnesium is mobile in the plant – when deficient, it moves from old leaves to young ones.

Symptoms (Mondal et al., 2020):

• Interveinal chlorosis on old leaves: veins remain green, but tissue between them turns yellow
• Yellowing often starts at the edges and spreads inward
• Leaves may become mottled, then brown and die
• Fruiting decreases, fruits become smaller

Causes: acidic soils, excess potassium (antagonism), magnesium deficiency in soil (especially on light sandy soils).

What to Do:

• Apply dolomite lime (contains magnesium) when liming – this solves the problem for several years.
• In acute deficiency – foliar spray with magnesium sulfate (10–20 g per 10 L water) or use potassium magnesium sulfate.
• Root drench with magnesium sulfate (30 g per 10 L water per 1–2 m²) is also effective.
• Maintain a potassium/magnesium balance: the optimal ratio in soil is about 3–5:1.

Calcium (Ca)

Role: building material for cell walls, stabilises membranes, participates in water transport. Calcium is practically immobile in the plant, so its deficiency appears on young organs.

Symptoms (Mondal et al., 2020; Kemble, 2022):

• Young leaves become small, crinkled, with curled edges
• On fruits – blossom‑end rot: a dark, water‑soaked spot at the fruit tip that then dries and becomes hard, leathery
• Root growth is impaired; roots become short and thick
• In severe deficiency – death of the growing point

Causes: uneven watering (calcium is transported only with water), excess nitrogen or potassium (antagonism), acidic soils (pH < 5.5).

What to Do:

• Ensure even watering – this is the key condition for calcium uptake.
• At planting, apply liming materials (dolomite, chalk) to maintain pH 6.0–6.5.
• For blossom‑end rot – foliar spray with calcium nitrate (10–15 g per 10 L water) on young fruits and leaves. Repeat 2–3 times at 5‑7‑day intervals.
• Do not overdo nitrogen fertilisers – they worsen blossom‑end rot.

Boron (B)

Role: cell division, pollination and fertilisation, carbohydrate transport, cell wall synthesis. Boron is critical for fruit set and seed formation.

Symptoms (Mondal et al., 2020; Kemble, 2022):

• Death of the growing point and apical buds
• Young leaves become thick, brittle, curled
• Flower and fruit set drop, poor pollination
• On fruits – cracks, corky tissue at the base or inside the fruit
• Internodes short, plant looks dwarfed

Causes: boron leaches easily from sandy soils; deficiency worsens in dry periods and with excess nitrogen or phosphorus.

What to Do:

• Apply boric acid or borax (2–3 g per 10 L water) as a spray at the start of flowering and repeat after 10–14 days.
• When preparing beds, you can incorporate boron into the soil (1–2 g boric acid per 1 m²) mixed with sand.
• Monitor soil pH (optimum 6.0–6.5) – boron is more available in slightly acidic conditions.
• Avoid overdosing: excess boron is toxic, symptoms include marginal necrosis of old leaves.

Iron (Fe)

Role: chlorophyll synthesis, respiration, nitrogen metabolism. Iron does not move through the plant, so deficiency appears on young leaves.

Symptoms (Mondal et al., 2020):

• Chlorosis of young leaves: veins remain green, tissue between them turns yellow (may become almost white)
• Differs from magnesium chlorosis in that it starts on upper, young leaves.
• In severe deficiency, leaves become whitish and die.

Causes: high soil pH (>7.0), waterlogging, excess phosphorus, manganese, zinc, or copper (antagonism).

What to Do:

• Spray with iron chelate (0.1 % solution) on leaves – the fastest method.
• Root application of iron chelate in dry form or with irrigation.
• If the problem is pH – acidify the soil (elemental sulfur, acid‑forming fertilisers).
• Improve drainage if waterlogging is present.

Manganese (Mn)

Role: enzyme activator, participates in photosynthesis (as part of the water‑splitting complex). Deficiency resembles iron deficiency but affects both old and young leaves.

Symptoms (Mondal et al., 2020; Kemble, 2022):

• Interveinal chlorosis on middle and upper leaves, veins remain green
• Leaves may become mottled, with necrosis as small spots
• With excess manganese (on acidic soils) – toxicity: old leaves turn brown, growing point may die

Causes: high pH (>7.0), salinity, excess phosphorus, iron, or zinc (antagonism).

What to Do:

• Spray with manganese sulfate (5–10 g per 10 L) before flowering and during fruit set.
• For soil deficiency – incorporate manganese sulfate into the bed (20–30 g per 1 m²).
• If pH is high – acidify the soil.
• Avoid excess lime – it reduces manganese availability.

Zinc (Zn)

Role: auxin synthesis, enzymatic processes, chlorophyll formation.

Symptoms (Mondal et al., 2020):

• Young leaves small, chlorotic, with wrinkled surface
• Internodes shortened, rosette appearance
• On old leaves – bronze spots, necrosis
• Fruits become deformed, fruit sets grow poorly

Causes: high pH, excess phosphorus or copper (antagonism), poor sandy soils.

What to Do:

• Foliar spray with zinc sulfate (2–5 g per 10 L) at the 3–4 true‑leaf stage and at the start of flowering.
• Zinc chelate can also be used for better absorption.
• Soil application – 20–30 g zinc sulfate per 1 m² mixed with sand.

Nitrogen "Lush Growth"

Symptoms:

• Vigorous, sprawling bush, leaves dark green, large
• Flowering delayed, fruit sets few or none
• If fruits do set, they are small, watery, with rough skin
• Plant is loose, more susceptible to diseases

Cause: excess nitrogen with insufficient phosphorus and potassium. Under nitrogen overfeeding, everything goes into vegetative mass, while the reproductive sphere is suppressed (Mondal et al., 2020).

What to Do:

• Stop nitrogen fertilisation as soon as flowering and fruiting begin.
• Shift emphasis to potassium and phosphorus fertilisers – they balance nutrition.
• Sometimes pinching the tips of shoots helps redirect energy to fruits.

Chlorosis on Over‑Limed Soils

Symptoms (Mondal et al., 2020): typical chlorosis of young leaves despite sufficient iron and manganese in the soil, but they are in unavailable forms due to high pH (>7.0–7.5).

What to Do:

• Check pH. If necessary – acidify (apply acid‑forming fertilisers, sulfur).
• Use iron and manganese chelates in sprays – they are absorbed through leaves regardless of soil pH.
• Do not apply lime without necessity.

Symptoms

• Plant flowers abundantly, but all flowers are "empty blooms" on long stalks
• Female flowers are absent or very few
• Bush is vigorous, leaves large, but no fruit sets

Physiological Causes

The ratio of male to female flowers in zucchini depends on three factors (Wien and Stützel, 2020):

1. Temperature. At high temperatures (above 30 °C during the day and above 20 °C at night), zucchini produces more male flowers. Female flowers are delayed or may not form at all (Wien, 2006; Wien and Stützel, 2020). This is the main cause of "empty blooms" in the middle of a hot summer.

2. Light regime. Long day length and high light intensity also stimulate male flowers, although this effect is weaker in zucchini than in cucumber (Nitsch et al., 1952; Matsuo, 1968).

3. Plant age. Young zucchini plants produce male flowers first. Female flowers appear later, when the bush has gained sufficient mass. This is an evolutionary mechanism: the plant must first build up resources for fruits.

In addition, excess nitrogen fertiliser in early stages can delay female flower formation, stimulating vigorous vegetative growth at the expense of reproduction (Ito and Saito, 1960; Wien and Stützel, 2020).

What to Do

• Be patient. If the plant is young (first 1–2 weeks of flowering), just wait – female flowers will appear later when the bush gains strength.
• Provide stable conditions. In hot weather (>30 °C), use shading, densify planting (but not too much), mulch the soil to lower root‑zone temperature.
• Balance nutrition. At the start of flowering, switch from nitrogen fertilisers to phosphorus‑potassium ones – they stimulate female flower formation. A foliar spray with monopotassium phosphate (15 g per 10 L water) can be given.
• Use ethylene‑based preparations. In commercial vegetable growing, ethephon (a female‑flower stimulant) is used, but this is not always available to amateurs. Instead, you can try spraying with a solution of succinic acid (1 g per 10 L) – it gently stimulates flowering.
• If the heat is prolonged, consider planting heat‑tolerant varieties – they have a genetically higher proportion of female flowers (Wehner et al., 2020; Rana, 2018).

Cause

At low temperatures (below 15 °C at night), the development of male flowers in zucchini is suppressed more strongly than that of females. As a result, the first flowers are female, and there is no pollen for pollination (Rylski and Aloni, 1990; Wien and Stützel, 2020). This is especially characteristic of some hybrids that tend to flower early.

What to Do

• Wait. As it warms up, male flowers will catch up and the problem will disappear.
• Hand pollination (see below) will not help if there are no male flowers. You can use preparations to stimulate male flowers, such as gibberellic acid (GA₃), but this is rarely used in amateur gardening.
• For the future, choose varieties with a more balanced initial flowering or grow under cover to mitigate cold stress.

Symptoms

• Fruit sets exist, but they turn yellow, shrivel, and drop
• Fruits grow unevenly – thicken at one end, become curved ("hooked")
• Fruits may be small and underdeveloped
• On cross‑section – uneven seed distribution

Physiological Causes

For normal fruiting, zucchini requires cross‑pollination by insects, primarily bees and bumblebees. Zucchini pollen is heavy and sticky; it is hardly carried by wind (Swiader, 1992; Free, 1970). In the absence of pollinators or under poor weather conditions, pollination does not occur or occurs only partially.

If insufficient pollen is deposited, the ovary initiates a programme of abortion – it turns yellow and drops. If pollination was only partial (e.g., pollen landed on only one of the three stigmatic lobes), the fruit grows crooked – seeds develop only in the pollinated part, while the remaining tissue does not grow (Swiader, 1992; Wehner et al., 2020).

In addition, pollen quality depends on temperature:

• At temperatures below 15 °C, pollen does not germinate
• Above 32–35 °C, pollen becomes sterile
• Optimal temperature for pollination – 20–25 °C

High humidity (>80 %) also hinders – pollen sticks together, bees do not fly.

What to Do

• Attract pollinators. Plant nectar‑bearing plants nearby (phacelia, calendula, oregano), set up water sources for bees, do not apply insecticides during flowering.
• Provide bee access. If you use covers (agrofabric, plastic), open them in the morning when bees are active.
• Perform hand pollination – this is the most reliable method for small plantings. In the morning (before 10–11 a.m.), pick a male flower, remove the petals, and gently touch the anther to the stigma of the female flower. You can also transfer pollen with a brush. One male flower can pollinate 2–3 female flowers.
• In windy and rainy weather, hand pollination is essential – bees do not work in such conditions.
• Do not spray plants with insecticides during flowering. If treatment is necessary, do it in the evening when flowers are closed, and choose products safe for bees.

Symptoms

• Young fruit sets (small fruitlets) turn yellow, soften, and rot right on the bush
• Fruits drop without signs of rot (dry abscission)
• Often fruitlets rot from the tip (floral end)

Physiological Causes

Rotting and dropping of fruit sets can be caused by several factors, often acting together:

1. Incomplete pollination. If the ovary was not fully pollinated, it begins to develop but then dies due to a lack of hormones produced by developing seeds. At first it may seem that the fruit is growing, but then it turns yellow and drops (Swiader, 1992).

2. Water stress. Lack of moisture or, conversely, waterlogging disrupts the transport of nutrients to young fruits. Calcium, critical for cell walls, is not delivered – cells break down, and rot begins (Mondal et al., 2020).

3. Excess nitrogen. Under nitrogen "lush growth," the plant directs resources to leaves, and fruits receive little nutrition. Fruit sets become weak and easily rot (Mondal et al., 2020).

4. Boron deficiency. Boron participates in cell division and seed formation. With its deficiency, fruit sets are underdeveloped, often die and rot (Mondal et al., 2020).

5. High humidity and cold. In damp, cold weather, fruit sets lie on wet ground – this provokes bacterial and fungal rot (Kemble, 2022).

What to Do

• Check pollination. If there are many fruit sets but only a few grow – pollination is poor. Pollinate manually.
• Remove excess moisture. Place straw, boards under fruits so they do not touch wet ground. Ventilate plantings, thin crowded bushes.
• Balance fertilisation. Start with phosphorus and potassium, exclude nitrogen during fruiting. Add boric acid (2–3 g per 10 L) in sprays.
• Water evenly, avoiding drying out and waterlogging.
• At the first signs of rot at the fruit tip – remove such fruit so it does not rot and infect neighbouring ones.
• If the problem recurs year after year, check soil pH and calcium and boron content.

What to Do

• Do not try to keep all fruit sets – the plant regulates the load itself.
• If you want several large fruits, leave no more than 3–4 on the bush at a time. Remove excess at the small‑fruitlet stage.
• If your goal is to obtain the maximum number of fruits (for preserving), harvest them young, not allowing them to overgrow. Then competition weakens, and new fruit sets set faster.

Symptoms

• Fruits hooked (curved like an arc or hook)
• Fruit tip swollen, base thin ("pear‑shaped")
• Fruit uneven in thickness along its length
• Often such fruits have uneven colour

Physiological Causes

The main cause of curvature and deformation is incomplete pollination (Swiader, 1992; Wehner et al., 2020). In zucchini, the female flower has a three‑lobed stigma. If pollen lands on only one of the three lobes, seeds develop only in one part of the ovary. In that part, the fruit grows, while in the unpollinated parts it does not. As a result, the fruit grows crooked because one side lags in development.

Other factors:

• Boron deficiency – boron participates in seed formation. With its deficiency, seeds set poorly, and the fruit grows unevenly (Mondal et al., 2020).
• Low temperature during pollination – slows pollen tube growth, making pollination incomplete (Wien and Stützel, 2020).
• Overheating – pollen becomes sterile, fruit sets either do not form or grow crooked (Wehner et al., 2020).
• Lack of moisture during ovary growth – cells do not receive enough water and grow unevenly.

What to Do

• Ensure full pollination. Attract bees or pollinate manually (see Chapter 5). Each female flower should receive enough pollen to pollinate all three stigma lobes.
• Apply boron during flowering. Spraying with boric acid (2 g per 10 L water) or soil application (1–2 g per m²) improves seed quality and fruit shape (Mondal et al., 2020).
• Maintain stable moisture – sharp fluctuations worsen deformation.
• Avoid overheating and overcooling during flowering – this reduces pollination quality.
• If deformation occurs every year, the variety may not suit your climate. Try more stable hybrids.

Symptoms

• Fruit widens towards the tip, narrows at the pedicel
• Often seen on the first fruits of the plant

Causes

This is a special case of incomplete pollination, but sometimes the cause is competition between fruits. The first fruit "takes" the bulk of resources; subsequent ones grow slower, and their shape may be distorted (Wien and Stützel, 2020). "Pear‑shaped" fruits are also typical for fruits set under low light or cold – cells grow unevenly.

What to Do

• Regularly remove overgrown fruits – they inhibit the development of others.
• Provide sufficient potassium and phosphorus nutrition – they improve fruit quality.
• If deformation is severe, remove such fruits to avoid wasting resources.

Symptoms

• Longitudinal or transverse cracks appear on the skin
• Cracks may be superficial (skin split but flesh intact) or deep
• Often cracks become entry points for rots and pests

Physiological Causes

Cracking is a classic result of sharp fluctuations in moisture (Mondal et al., 2020; Swiader, 1992). If, after a dry period, you give abundant watering or heavy rain falls, fruit cells begin to rapidly absorb water. The skin, having lost elasticity during the drought, cannot withstand the internal pressure – it bursts.

Additional factors:

• Thin‑skinned varieties are more prone to cracking
• Sharp temperature fluctuations (hot day – cold night) increase tissue tension
• Calcium deficiency reduces cell wall strength (Mondal et al., 2020)

What to Do

• Water regularly, avoiding drying out and subsequent flooding. Better more often, but in smaller amounts.
• Mulch the soil – this smooths out moisture fluctuations.
• Do not water with cold water in heat – this causes thermal shock and may trigger cracks.
• For thin‑skinned varieties, use drip irrigation – it is the most stable.
• With blossom‑end rot (calcium deficiency), cracks often combine with necrosis of the fruit tip. In this case, spray with calcium nitrate (10–15 g per 10 L).
• If cracking becomes widespread – harvest all damaged fruits, even if immature, to stop moisture loss and prevent rot.

Symptoms

• Pale, whitish or light‑brown spots appear on fruits
• Spots are sunken, skin becomes wrinkled, papery
• Later, spots may rot because tissues die

Physiological Causes

Sunscald occurs when the fruit is exposed to direct sunlight without leaf protection, especially in hot weather (above 30 °C) (Mondal et al., 2020; Narina, 2016). Cells overheat, protein denatures, tissues die. Fruits with dark green skin are more often affected – they absorb more heat.

Scald can also be caused by water droplets left after watering or rain – they act as lenses, focusing sunlight. However, the main cause is insufficient leaf cover.

What to Do

• Provide sufficient leaf mass. Do not remove leaves unless absolutely necessary – they shade the fruits.
• Shape the bush properly. If the variety is prone to scalding, direct the vines so that leaves cover the fruits.
• Use shading during the hottest hours (shading net, light shade from tall crops).
• Water only at the root or early in the morning so that droplets dry before the sun is strong.
• When choosing varieties, prefer light‑green or yellow fruits – they heat up less.
• If scalding has already occurred, remove damaged fruits – they will not recover and may rot.

Symptoms

• When cut, cavities are visible inside, especially in the central part
• Flesh may be loose, with voids around the seed cavity
• Fruit is light, sounds hollow when tapped

Physiological Causes

Hollow cavities form when inner tissues grow more slowly than outer ones. This occurs during rapid fruit expansion, when pericarp cells stretch but the seed cavity does not fill in time (Sinnott, 1939; Kano, 1993; Wien and Stützel, 2020).

This is most often associated with:

• Incomplete pollination (lack of seeds, which produce hormones for tissue development)
• Excess nitrogen and rapid fruit growth with insufficient potassium
• Uneven watering – rapid volume increase after drought
• Overgrowing of fruits on the bush
• Genetic predisposition in some varieties

What to Do

• Ensure good pollination – fully developed seeds prevent hollow cavities.
• Balance nutrition – during fruiting, give potassium and phosphorus, limit nitrogen.
• Maintain stable moisture – avoid sharp fluctuations.
• Harvest fruits on time, do not let them overgrow.
• When establishing new plantings, choose varieties with dense flesh that are resistant to hollow cavities (especially for hot climates).

Symptoms

• Fruits become very large, but skin is rough, hard
• Skin often darkens, becomes rough
• Flesh becomes watery, loose, with large hard seeds
• Taste is lost, fruit becomes bland or insipid

Physiological Causes

Zucchini is a crop whose fruits are edible only when immature. When overgrown, the cell walls of the fruit lignify, accumulating cellulose and lignin. Reserve substances turn into starch, sugars decrease. This is a natural ripening process, but in zucchini it makes the fruit inedible (Rana, 2018; Kemble, 2022).

In addition, on old plants, when the leaf apparatus ages, fruits often overgrow because the plant cannot spend resources on forming new fruit sets – it "ripens" the already set fruits.

What to Do

• Harvest fruits regularly, preventing overgrowth. Optimum size – 15–20 cm, diameter 4–6 cm.
• Harvest every 2–3 days at peak fruiting. A missed fruit quickly loses market quality and inhibits the formation of new ones.
• When harvesting, cut the stalk with a sharp knife, do not tear it (damage may cause rot).
• If a fruit has overgrown – remove it (to compost) so it does not take resources from young fruit sets.
• Maintain a healthy leaf apparatus – remove ageing leaves to improve light and ventilation.

Symptoms

• Fruits have a bitter, unpleasant taste
• Bitterness may be uniform or intensify toward the fruit end
• Even with good appearance, the fruit is inedible

Physiological Causes

Bitterness in zucchini fruits is caused by cucurbitacins – a group of bitter compounds that protect the plant from animal feeding (Wehner et al., 2020; Rymal et al., 1984). In wild species, cucurbitacins are always present. In cultivated varieties, their content is minimised, but under stress, synthesis can resume (Mondal et al., 2020).

Main stress factors:

• Drought – the most frequent trigger (Wehner et al., 2020)
• Overheating and intense sunlight
• Sharp temperature fluctuations
• Potassium deficiency or other nutritional disturbances
• Root damage (by pests, diseases, mechanical)
• Genetic failure – sometimes a mutation leads to bitter fruits on individual plants (Herrington, 1983; Rymal et al., 1984)

Important! In rare cases, bitterness can be so strong that it causes poisoning (nausea, diarrhoea). Such fruits should not be consumed.

What to Do

• Provide plants with stable watering – this is the main prevention of bitterness.
• Avoid overheating and drying out (see Chapters 2 and 3).
• Fertilise with potassium during fruiting – potassium reduces cucurbitacin accumulation.
• If bitterness appears on individual fruits, check the variety – cross‑pollination with ornamental pumpkins may have occurred. Remove such fruits.
• When harvesting, taste the first fruit. If bitterness is present – check other plants; the problem may be local.
• Bitter fruits should not be used even for animal feed – they are toxic.