Ascochytosis of cucumber: treatment and prevention of fungus

Cucumbers belong to the Cucurbitaceae family, all members of which are susceptible to various fungal infections. One of these is cucumber stem rot (black mycosphaerella rot), caused by the pathogens Didymella bryoniae and Ascochyta cucumis, which are active at different stages of development. The infection is characterized by its slow progression, primarily affecting greenhouse and hothouse plants during the fruit formation and growth phase.

Reasons for occurrence

The disease is caused by fungal infection of the seed, where the pathogen remains dormant for a long time. It is unaffected by treatments and heating to 80°C, which kills most pathogens.

During the seed and seedling stages, up to the second leaf stage, the pathogen is not released. It emerges during the growing season. Initially, it emerges from the tissues of the root collar and forms a slowly growing mycelium that spares the vascular system. Then, colonies begin to emerge from the stem tissues, growing more rapidly. The infection gradually spreads throughout the stem, leaves, and fruit. Spores are carried by the air to neighboring plants.

The fungus thrives in temperatures ranging from +10°C to 32°C and humidity from 20% to 100%, but develops and spreads more rapidly under certain conditions. Rapid infection spread is facilitated by humidity levels above acceptable limits, moisture retention (dew, water) on the plant surface, high temperatures (20-25°C), nematode infestation, dense plantings, and overwatering. Infection progression is significantly slowed by alternating warm, rainy weather and drought, and ceases at temperatures above 35°C.

Ascochytosis affects only weak plants and those on which dry and yellow leaves, dying shoots, and remaining petioles are not removed in a timely manner.

Factors that reduce cucumber resistance to fungal infection:

• Cold water used for watering increases the aggressiveness of the pathogen - the plant will wither after 1-2 weeks;
• significant reduction in temperature in the greenhouse;
• the chemical composition of the soil is unsuitable for growing cucumbers (the balance of microelements is disturbed);
• organic matter (manure, compost) of poor quality;
• incorrect crop rotation;
• presence of infections in the soil.

You may be interested in:

Maximum cucumber harvest thanks to super-fertilizing in August
Days in August are still warm, but at night the temperature can drop to 8-10°C. Dew and fog in the mornings do their job,…Read more…

How does it manifest itself?

The disease is difficult to spot in the early and mid-growing stages—it only becomes apparent around the fruiting period. It's almost never seen on young shoots. Symptoms vary across plant parts and at different stages of growth.

Stems

At the beginning of the growing season, water-soaked, round or oval, grayish-green spots appear on the stem. They then turn brown and, as they dry out, turn white-gray. The upper stem tissues crack, and a brown or milky exudate appears.

Vascular tissue is not affected at this stage, so the diseased bush continues to grow and develop. Small black spots (pycnidia) then appear on the surface of the affected areas. The spots grow rapidly, gradually covering the entire stem.

Leaves

Symptoms appear on the leaf blade during the fruiting phase. The edges of the leaf become noticeable.

The affected leaf gradually turns brown, then light yellow, and pycnidia appear, either randomly arranged or in circles. If these leaves are not removed, they will quickly dry out.

Fruit

Ascochyta blight manifests itself on fruits in various ways. Three forms can be distinguished:

Form I. Initially, the top of the fruit (or at the base) becomes "cooked," shrinking while remaining hard, with black spots appearing on it. Then the entire fruit turns black. Gradually, it mummifies or begins to decompose (as with black rot). The surface of the testes cracks or small ulcers form, from which gum oozes.

Form II. Dry, ulcerated lesions, 3 to 5 mm in size, appear on the cucumber, densely covered with pycnidia.

Form III. The flesh of the cucumber becomes "rusty." First, the top of the fruit becomes flabby, then a "rusty" spot develops in the flesh, and slime appears within it. With this form of infection, secondary bacterial rot develops, affecting the entire fruit.

Control measures

Fighting the fungus is difficult because no treatment is effective during the dormant stage and until it manifests itself. To prevent infection, preventative measures are taken, and if the infection does manifest, other methods are used to combat cucumber ascochyta blight, including various chemical and biological treatments.

Prevention

The pathogen's developmental characteristics make it resistant to fungicides and biological agents before and during the initial stages of vegetation. Reduce the likelihood of occurrence diseases Preventive measures and proper agricultural practices will help:

• watering should be done in the evening - the air humidity in the greenhouse decreases during the day and the rate of infection development decreases;
• use settled water that has warmed up during the day to avoid a sharp change in temperature between the soil and water, which leads to the appearance of root (tap) rot and weakening of the plant;
• observe the watering regime and norm - the soil should not be over-dried and not “wet”;
• maintain an optimal temperature in the greenhouse, avoiding sudden changes;
• remove affected leaves on the lower tier as soon as black spots appear on them - this will prevent the spread of mycelium;
• use varieties resistant to ascochyta blight, for example, Izobilny, Leningradsky Teplichny, Amazonka, Luzhok F1, Romance F1, Prolific;
• cover (mulch) the soil with film when growing seedlings to reduce the risk of the stem form of the disease, which helps to extend the growing season;
• change crops in the greenhouse;
• in early spring, sow green manure and dig up the soil along with it a month before planting the seedlings;
• heat treatment of the seed material will not kill the pathogen, but will reduce its aggressiveness and delay the onset of the disease to a later date;
• maintain the recommended distance between bushes.

You may be interested in:

Fusarium wilt of cucumbers: causes and treatment methods
Fusarium wilt of cucumbers is a common occurrence caused by infection of the vascular system…Read more…

Use of biological drugs

The activity of ascochyta fungi is suppressed by soil microorganisms contained in Trichoderm SP, Trichoderm Veride 471, Trichophyte, and Trichoflor. Their use delays the onset of the disease by 3-4 weeks, reduces the severity of the disease by 14%, and decreases the number of pathogen colonies in the soil. Apply according to the instructions for treating plants, soil, and seed:

• the soil is watered when digging in the spring or only the hole is watered, using 30-40 ml/m², after 2-3 weeks, the roots are watered;
• soak the seeds (25-40 min), root system (15-30 min), consumption 30-50 ml/l;
• Spray once a week (25-50 ml/10 l) during flowering, fruit setting and fruiting.

Other biological agents:

• Fundazol – active against fungus on plants, in soil (10 g/10 l);
• Trichocin SP – used to treat the soil three days before planting seedlings and a week after (6 g/10 l), repeated in summer and autumn when the harvest is collected;
• Topaz - treat the entire above-ground part with a solution (2 ml/10 l) at the very beginning of the disease, repeat after 7-14 days.

Use of chemicals

Sometimes biological treatments fail to slow the spread of infection, so chemical agents must be used. These are used both prophylactically and when symptoms appear.

Application options:

• 2-5% formalin solution for treating greenhouses and structures, consumption rate – 1 l/m²;
• 0.7-1.0% solution of Bordeaux mixture, 0.3% suspension of copper oxychloride, copper sulfate in solution (5 g/10 l) + urea (10 g/10 l) - one of the agents is sprayed on the bush for the foliar form of the disease, repeating the treatment 3-4 times every 10-12 days;
• the affected area on the stem is sprinkled (lubricated) with a powder of chalk and copper sulfate (1:1);
• A 0.1% solution of Saprol, used to treat powdery mildew, reduces the rate of infection;
• the fungicide Rovral 1% is used to disinfect seeds and soil, and to spray the plant (1 g/1 l);
• Topsin M – cultivates the soil during crop rotation.

https://youtu.be/mlpqaCqj0Cw

Proper cucumber cultivation techniques and preventative measures can either prevent ascochyta blight or delay its onset, allowing the crop to bear fruit for a longer period.