Ascochyta blight of cucumber in greenhouse: biology and methods of plant protection

Ascochyta blight of cucumber in protected ground is caused by the fungus Ascochyta cucumeris. In addition to asexual pycnidial sporulation, this pathogen produces numerous perithecia with ascospores (the sexual stage), which indicates active morphogenesis processes. The fungus is a facultative parasite and primarily infects weakened plants. With proper care, the cucumber continues to provide a harvest even when signs of the disease are present.

Pathogen biology and risk factors

The main source of infection is contaminated seed. In studies from 1996–1998, the contamination rate of various batches of cucumber seeds of the Estafeta and TSKhA-575 cultivars ranged from 12% to 60%. The fungus also survives on greenhouse structures and plant residues in the soil, from where it can spread through the air. According to 1994 research data, the Izobilny 131, Plodovity 147, Leningradsky teplichny 23, and Dlinnoplodny 1294 cultivars exhibit relative resistance to Ascochyta blight.

The susceptibility of the cucumber to the disease increases sharply when agronomic practices are violated. Key risk factors include root damage by the root-knot nematode, high-density planting, and excessive irrigation. In addition, plants on which yellowing leaves, dying shoots, and remnants of leaf petioles remain for a long time are more severely affected.

Protection measures in the greenhouse

During the growing season, the mycelium is located inside plant tissues, making it virtually inaccessible to fungicides and biological agents. A radical solution would be the production of clean seed, but this is difficult to implement due to the complex biology of the pathogen. Foreign hybrid seed has low contamination rates, but its biological potential in our greenhouses is often not fully realized due to microclimate mismatches.

To control the disease, the experience of Dutch greenhouse growers is used, who change the irrigation regime when there is a risk of epiphytotic spread. Irrigation is stopped completely during the day, and the crop is irrigated 1–2 times at night. This allows for a reduction in daytime Relative humidity air">relative air humidity and slows down the rates of infection development. In trials, the Alirin-B preparation also showed a high level of efficacy against Ascochyta blight.

• Hybrid seed contamination — 12–60%
• Disease delay with Trichoderma harzianum — 18–25 days
• Reduction in the degree of disease development — by 14%
• Application rate of Alirin-B — 60–80 kg/ha
• Consumption of 2–5% formalin solution — 1 L/m²
• Cost of biological soil treatment — 20–25% of the cost of disinfection

To improve the quality of seed material before sowing, thermal seed treatment is conducted. The thermotherapy regime includes three sequential stages:

1. Drying the seed for 2 days at a temperature of 35 °C.
2. Heating for 3 days at a temperature of 50–52 °C.
3. Heating for 1 day at a temperature of 80 °C.

This regime completely destroys accompanying pathogens on the seed. The Ascochyta blight pathogen is less sensitive to this procedure; however, after heating, disease symptoms appear 8–10 days later, and the aggressiveness of root rot is reduced.

During the growing period and upon crop termination, a complex of agrotechnical and chemical measures is performed:

• Timely removal of affected leaves from the lower tier. This should be done only after the pycnidia have turned black (matured), which restricts the spread of the mycelium up the stem.
• Mulching the soil with polyethylene film from the moment of planting transplants until the end of the growing season to protect against the stem form of the disease.
• Observance of crop rotation and sowing of rye or oats as green manure. 15–20 days before planting transplants, the green manure crop is shredded and incorporated into the soil along with manure or humus.
• Wet disinfection of greenhouse structures in the autumn with a 2–5% formalin solution, as well as steaming or fumigation of the soil.
• Treatment of growing plants with benzimidazole group preparations (fundazol) to inhibit parasite growth.

In steamed and fumigated soil, the natural microbial coenosis is depleted; therefore, it is necessary to artificially introduce beneficial microflora. Common soil microorganisms are active against Ascochyta blight: Chaetomium globosum, Chaetomium lentum, Bipolaris sorokiniana, Mucor sp., Trichoderma sp., and Aspergillus sp. Prophylactic application of Trichoderma harzianum delays disease development and reduces the degree of its manifestation. If Chaetomium globosum and C. lentum dominate the soil population, the pathogen count decreases.

Maintaining high biodiversity and timely prevention allow for maintaining satisfactory phytosanitary conditions without annual soil steaming. Switching to disinfection once every two years provides the farm with significant cost savings. This approach preserves the beneficial soil microflora and reduces production costs.

Practical control measures for cucumber ascochyta blight

When ascochyta blight transitions to the foliar form, immediate treatment with copper-based preparations or tank mixtures with nitrogen fertilizers is required. Spraying stops the spread of infection across the foliage and preserves the photosynthetic surface. Proven treatment schedules with an interval of 10–12 days are used.

• Bordeaux mixture concentration — 0.7–1.0 %
• Copper oxychloride suspension concentration — 0.3 %
• Dosage of copper sulfate for the mixture — 5 g per 10 l of water
• Dosage of urea for the mixture — 10 g per 10 l of water
• Frequency of spraying per cycle — 3–4 times

The stem form of ascochyta blight requires local surgical intervention and stimulation of plant regeneration. The affected root collar and lesions on the stems are treated with contact mixtures. After this, new root growth is induced to replace the damaged underground part.

1. Dust or smear the affected stem areas with copper-chalk powder by mixing copper sulfate and chalk in a 1:1 ratio.
2. Treat the root collar with the same mixture and mound soil over the part of the stem above the affected zone to rejuvenate the plant.
3. After the formation of adventitious roots, completely remove and destroy the old root system along with the affected stem section.

Olive leaf spot (cladosporiosis) and anthracnose of cucumber

Olive leaf spot (cladosporiosis) is caused by the pathogen Sadozrotit sisiteipit EP its Ag and is most dangerous for plastic greenhouses. The disease affects leaves, stems, and fruits, causing the death of vegetative mass and deformation of the cucumbers. In cool and humid periods, the infection can destroy a significant portion of the marketable harvest.

Symptoms of cladosporiosis differ by plant organ. Angular, light-brown spots with a yellow halo and an olive-colored mold form on the leaves; the tissue in these places quickly drops out, and rosettes of 3–7 deformed small leaves form at the tips. Dry ulcers with a grayish-olive mold appear on the stems. Oily depressions form on the fruits, developing into ulcers up to 2.5–3 mm deep with droplets of thick yellow liquid; meanwhile, young cucumbers become twisted and stop growing.

The pathogen is quickly transmitted by airborne droplets, developing most actively near cold outer glazing. The source of primary infection is conidia (measuring 10–25 x 3–6 µm) overwintering on plant residues, elements of the greenhouse structure, and trellis wire. On biofuel, the disease develops less severely than in winter glass greenhouses.

For protection against olive leaf spot, it is necessary to strictly monitor microclimate parameters and conduct timely treatments. Genetic resistance to the pathogen is determined by the dominant monogene Ssi, which is carried by the hybrids E, Hercules, and Taiga. The protection system is built on disinfection and maintaining optimal humidity.

• Maximum air humidity — no more than 80 %
• Depth of fruit flesh damage — 2.5–3 mm
• Delay in disease development with biological protection — 5–12 days
• Incubation period of the pathogen — 6–7 days
• Bordeaux mixture concentration — 0.7–1.0 %
• Copper oxychloride suspension concentration — 0.3–0.4 %

Phytosanitary measures include mandatory cleaning of greenhouses from plant residues at the end of the season followed by disinfection of the premises. For prevention, the crop is sprayed with the biological product Pseudobacterin-2, which shifts the timing of symptom manifestation and protects plants during the first two months of fruiting. When signs of the disease appear, the plantings are treated 3–4 times per season with an interval of 10–12 days with Bordeaux mixture, a suspension of copper oxychloride, or Oxyhom.

Anthracnose, caused by the fungus Colletotrichum lagenarium (Pass.) Ell. et Halst., is harmful everywhere, especially in plastic greenhouses and in open ground in the south. The disease affects all above-ground organs of the cucumber and other cucurbits, reducing yield and the content of ascorbic acid in the fruits. At the seedling stage, the infection manifests as brown sunken spots in the root collar area, which quickly encompass the entire stem and cause the death of young plants.

Cucumber anthracnose: symptoms, biology, and control measures

Anthracnose affects cucumber leaves and stems, manifesting as circular yellowish spots ranging from 3 mm to 3–4 cm, which are most often located along the edges of the leaf blade. With severe development of the disease, these spots merge, and the affected tissue tears, forming slit-like holes. Diseased leaves stop functioning properly, and damaged stems break easily. Subsequently, the infection moves to the cucumbers: sunken, elongated, light-brown spots form on them, under which the mycelium penetrates 3–4 mm into the depth of the fruit tissues.

On all infected cucumber organs, the fungus develops sporulation in the form of numerous pale-pink cushions, which are arranged in concentric circles or merge into a solid coating. The conidia of the pathogen are single-celled, colorless, elongated-oval in shape with granular plasma, measure 11.5–20 x 3.5–6.5 µm, and are glued together by a special substance. The fungus overwinters on plant debris in the form of sclerotia and pseudopycnidia, and the source of infection can be seed collected from diseased plants.

Environmental factor	Optimal parameters for anthracnose development
Temperature range	4–30 °C
Air humidity	90–98 %
Incubation period	4–7 days

To protect against anthracnose, it is sufficient to carry out a standard set of preventive measures, including disinfection of greenhouse structures and soil. Biological methods show good results. Seed soaking in the culture liquid of the bacterium Pseudomonas mycophaga (strain D1), isolated from the rhizosphere of cucurbit crops, significantly reduces plant infestation and increases yield. Application of the actinomycete (strain 175) to the soil also reduces the spread of the disease, and when the first symptoms appear on leaves and stems, it is recommended to carry out spraying with biological products Alirin-B and Alirin-S.

• Application rate of TMTD for seed treatment — 4 g per 1 kg of seed
• Reduction of seed infestation by bacteria of strain D1 — from 34.1–37.3 % to 4.1–6.0 %
• Yield increase from the application of strain D1 — 33–45 centners/ha
• Reduction of infestation when applying the actinomycete — down to 7.3–13.0 %
• Yield increase from the application of the actinomycete — 31–36 centners/ha

Chemical protection begins with pre-sowing preparation: seed is treated with the product TMTD (thiram) or soaked before sowing in a solution of Immunocytophyte. In smallholdings, anthracnose is controlled by root irrigation with a Bordeaux mixture solution.

1. Water the plants abundantly with clean water before chemical treatment.
2. Prepare a 0.5–1 % solution of Bordeaux mixture.
3. Carry out root irrigation with a consumption of 0.5–1 L of solution per plant so that the main part of the liquid enters the root zone and only a small amount reaches the stem.
4. If necessary, repeat the treatment 2–3 times a week.

If symptoms of anthracnose appear on the leaves, the growing plants are sprayed with a 1 % solution of Bordeaux mixture or a 0.3 % suspension of copper oxychloride. Sulfur products (Thiovit, colloidal sulfur, Kumulus) and fungicides from the strobilurin group — Quadris and Strobi — are also effective.

Powdery mildew: harmfulness and disease manifestations in greenhouses

Powdery mildew is a harmful disease of cucumber that affects plants in glass and plastic greenhouses. Significantly fewer fruits are set on severely stunted plants, although there are no signs of the disease on the fruits themselves. As the infection develops, all leaves become covered with a powdery coating, plants lose water intensively and dry out quickly. In northern regions, the cucumber is parasitized mainly by the fungus Erysiphe cichoracearum, whereas the pathogen Sphaerotheca fuliginea is a typical representative of southern regions.

In greenhouses, the disease affects cotyledons and leaves. At first, a fluffy white coating appears on the upper side of the leaf blade in the form of round spots. Over time, these spots merge and gradually darken, and the leaves themselves deform, acquiring a wavy surface. Later, the characteristic coating spreads to the underside of the leaves, petioles, and stems. The conidia of the pathogen are barrel-shaped and measure 30–40 x 15–20 µm; conidiophores bear one apical spore each.

The sexual stage of the fungus develops on the mycelium exclusively in open-field conditions. The morphology of the cleistothecia differs depending on the species of the pathogen:

• Erysiphe communis (syn. Erysiphe polygoni) forms cleistothecia 65–180 µm in diameter with simple or branched appendages at the apex, which intertwine with the mycelium. Inside, there are 2–8 egg-shaped or round asci on short stalks with 3–6 (rarely 2 or 8) ellipsoidal ascospores measuring 19–25 x 10–14 µm.
• Sphaerotheca fuliginea (in the sexual stage Sphaerotheca fuliginea f. cucurbitae) forms spherical cleistothecia 50–100 µm in diameter with twisted, colorless appendages. They contain only one ascus with 2–8 single-celled ellipsoidal ascospores measuring 20–25 x 12–15 µm.

Routes of infection spread and conditions for disease development

True powdery mildew is capable of rapidly destroying cucumber plantings in protected ground. An agronomist should look for the primary foci of the disease near vents, doors, and broken glass. In the conditions of Central Russia, the infection appears in greenhouses within 3–4 days after heavy rains in late April or early May. Pathogen spores are carried by the wind from southern regions, where the growing season begins earlier. In the south of the country, conidia infect cucumber continuously throughout the entire summer, as well as in winter and in the transitional crop cycle.

The main source of internal infection is the lack of a sanitary break between the autumn and winter-spring crops. During this time, pathogen spores manage to transfer into the nursery areas. If the greenhouse operation is small and the break between rotations is long, the fungus survives on the residues of pumpkins and squashes growing in adjacent plots (in the form of ascospores in overwintered cleistothecia). An additional reservoir of infection is provided by weeds around the greenhouses — plantain (Plantago spp.) and sow thistle (Sonchus asper).

The genetic resistance of hybrids helps to contain the pathogen without unnecessary treatments. The recessive genes rt-1 and rt-2 provide protection for the cultivar Matsu a5B tat, and the recessive gene mlo-A is present in the VIR accessions P.I.200815 and P.I.200818. Another recessive gene controls the resistance of cucumber seedlings (cotyledons and hypocotyl) and is characteristic of the cultivar resistant to downy mildew 4t. This explains the fact that plants resistant to downy mildew often demonstrate resistance to powdery mildew as well.

• Temperature for conidia germination — 16–20 °C
• Duration of the incubation period — 3–5 days
• Sanitary break between crops — at least 3 weeks
• Night temperature in the greenhouse — not lower than 17 °C
• Day temperature on sunny days — not higher than 30 °C
• Water temperature for irrigation — 20–22 °C

Agrotechnical, biological, and chemical control measures

Disease prevention is based on strict control of greenhouse hygiene and the maintenance of an optimal microclimate. Agronomists are advised to prioritize resistant and less susceptible hybrids with dark green leaves — an increased chlorophyll content improves their resistance. High resistance is shown by the hybrids F1 Alyans, Turnir, Strema, Regata, Semkross, Lastochka, Golubchik, and the cultivar Feniks, while the hybrid F1 Tayga is distinguished by its tolerance to the pathogen. Mandatory practices remain the complete cleaning of cultivation areas from plant residues and their disinfection.

To control the disease in private smallholdings and at the first signs of infection, folk methods, biological agents, and specialized chemicals are used. The biological products Planriz and Alirin are effective against primary symptoms of the disease. Among chemical agents, Quadris, Strobi, and Topaz are recommended for preventive use, while sulfur-based preparations show maximum results when treating already emerged foci.

1. Dilute one bucket of rotted cow manure with five buckets of water.
2. Let the mixture steep for 3–5 days.
3. Before use, strain the infusion thoroughly and dilute it with clean water in a 1:3 ratio.
4. Spray the plants over the leaves at intervals of 5–7 days (a triple treatment of lightly infested plants reduces disease development by 2–2.5 times).

Preparation / Agent	Working concentration	Application schedule and specifics
Baktofit	0.2%	Spraying at intervals of 10–12 days. Waiting period — 1–2 days.
Pseudobacterin-2	According to instructions	Spraying during the growing season.
Planriz, Alirins	According to instructions	Applied upon the appearance of primary disease symptoms.
Quadris, Strobi, Topaz	According to instructions	Recommended for preventive use before symptoms appear.
Bayleton, Cuproxat	According to instructions	Spraying upon detection of isolated foci.
Thiovit, Kumulus, colloidal sulfur	According to instructions	Applied strictly after the appearance of the first disease foci.
Immunocytophyte	According to instructions	Used for treatment to increase plant resistance.

Pathogen — Pseudoperonospora cubensis Rostovtsev. Harmfulness. An extremely harmful disease capable of destroying all plants in a greenhouse in a short time. The greatest damage is noted in the second half of summer, although the disease also develops perfectly in winter during the transitional rotation in southern regions.

Symptoms. Initially, oily yellowish-green spots appear on the upper side of the leaves. Later, only the petioles remain on the surface of the spots. Loss of foliage delays the process of fruit set and their normal development. Mature fruits are poorly colored and tasteless. In southern Russia in recent years, cucumber death has been caused by leaf spots, which represent a latent form of downy mildew. At first, small, vein-limited chlorotic spots form on the leaves. Increasing in size and merging, they completely cover the leaf blade, which turns lemon-yellow. Sometimes the spotting manifests as a mosaic resembling a viral infection. In high humidity, the spots are covered with exudate on the underside and take on an oily appearance. Chloroses and necroses spread, causing leaf dieback tier by tier; if the growth point is affected, the plant dies. Such signs are often mistakenly identified as symptoms of viruses, bacterial diseases, or a lack of magnesium and iron in plants. On the winter-spring cucumber crop, this form is detected 30–40 days before the appearance of typical signs of downy mildew with zoosporangia, while in the autumn crop, zoosporangia do not form at all until the end of the growing season.

Source of infection. Symptoms of the latent form of the disease appear simultaneously and evenly throughout the cucumber planting area, which is a consequence of seed infection. It has been established that the pathogen can persist as mycelium in the seed. Infected plant residues with oospores are also capable of overwintering, and in the spring, at a temperature of 15-20°, they germinate into primary zoosporangia, from which zoospores emerge. The latter are capable of infecting plants from the 3-4 leaf stage until the end of the growing season.

During the growing season, pathogen zoospores are easily transported by water droplets from open soil to greenhouses and vice versa, which ensures, on the one hand, a multiple increase in the number of infected plants, and on the other, the persistence of the pathogen during unfavorable periods.

Biology of the pathogen. Zoosporangiophores emerge from the stomata in clusters of 2-7, less frequently singly through the ruptured cuticle; they are swollen near the base, irregularly dichotomously branched at the top, with terminal branches departing at right angles. The zoosporangia are oval or ovoid, grayish or light violet in color, 20-25 x 16-20 µm.