Diagnostics and plant protection methods against root rot of cucumber

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Pythium debaryanum mainly affects the underground organs of cucumber in the seedling stage. The affected tissue turns brown and rots, the plants wilt, and the cotyledons and first leaves turn yellow. In addition to cucumber, this pathogen is capable of infecting a very wide range of host plants. P. debaryanum has a highly branched mycelium. It does not form zoospores, and the zoosporangia germinate mainly by means of a germ tube. The fungus is capable of parasitizing the seedlings of 150 plant species, including cucurbits and citrus crops. Parasitic nematodes facilitate the spread of phytopathogen spores in the soil. This fungus can form mycorrhiza with the roots of many plants.

P. arrhenomanes affects more than 80 species of plants. It is particularly harmful to cucumber, tomato, and soybean. The plant is most vulnerable before the shedding of the cortex; during this period, the primary cortex dies off and loses its protective properties. In the affected tissues, reproductive organs of the fungi are formed. After the formation of the secondary cortex, which in most plants forms at the age of two true leaves, the resistance of the seedlings is restored.

Resistance of cucumber to root rots. The harmfulness of root rots decreases or becomes insignificant when growing cucumber transplants on rootstocks resistant to this disease. Semi-cultivated species of squash are most often used as rootstocks — Cucurbita ficifolia and Lagenaria siceraria. They are resistant to low temperatures and tolerate excess soil moisture well. The best method that ensures the highest percentage of plant survival is grafting by tongue approach. Grafting using this method should be carried out when the rootstock is in the cotyledon leaf stage and the scion is in the stage of the first true leaf.

There are known cucumber hybrids that are weakly affected by root rots, for example, F1 Taiga and F1 Hercules, as well as tolerant hybrids F1 Gepard and F1 Mazay.

The application of certain microelements to the soil as a top dressing (MnSO4 and ZnSO4 at 0.25 g per 10 l of water) increases the resistance of cucumber plants to root rot and contributes to an increase in yield (Pidoplichko, 1968). MEASURES FOR PROTECTION AGAINST ROOT ROTS. Agrotechnical methods. To reduce the harmfulness of the disease, optimal conditions are created and a complex of preventive measures is performed, aimed at reducing the pathogen pool (thorough removal of plant residues, removal of the topsoil layer, disinfection of the soil and greenhouse structures, seed treatment or heating, etc.).

For the preparation of peat-humus pots, sod, humus, and peat soil in equal parts (1:1:1) or composted mixtures that have undergone biological sterilization for 3-5 years or have been pre-steamed (sterilized) are used. Disinfection of transplant mixtures, pots, basic soil, and soil for dressing is a mandatory measure. The use of tray technology with mineral wool plugs and cubes, as well as new sterile mats, allows avoiding plant infection through the substrate.

It is undesirable to use the same soil continuously, because many pathogen sclerotia accumulate in it. Plants are watered with warm water at a temperature not lower than the air temperature in the greenhouse. Soil moisture is regulated so that it does not exceed 85 % of the total water capacity. The soil temperature throughout the entire growing season should be in the range of 20-26°.

Substrate salinization should not be allowed, as this weakens the plant's root system.

Grow cucumber transplants on rootstocks resistant to root rots.

When the first signs of root rot appear, rejuvenate the plants: the stem is lowered to the soil to form a new root system. In this case, fresh soil must first be poured onto the stem to just cover it. Only after the appearance of new roots (in 10-15 days) is additional soil added. Before laying the stems on the soil, biological or chemical fungicides are applied.

Biological agents. To protect plants from root rots of various etiologies, several effective biological preparations are recommended: pseudobacterin-2, bactofit, and trichodermin. Biopreparations are most effective against Rhizoctonia rot, to a lesser extent against Pythium species and southern late blight, but are practically ineffective against Fusariums. It is advisable to apply bacterial preparations to the seed surface before sowing. One of the factors limiting the accumulation of root rot pathogens in the soil is the use of the biopreparation trichodermin, which contains an antagonist fungus. It is better to apply trichodermin to moist soil a few days before sowing seeds or to coat the bases of the stems with a paste made of the preparation with an adhesive during the planting of transplants in their permanent place. For example, applying trichodermin to the soil in greenhouses of state farms in the Moscow region reduced the number of plants that died from root rot by 3 times, which increased the yield per 1 m² by 3 kg. Currently, pseudobacterin-2, bactofit, and trichodermin are recommended for protecting plants from root rots. Bacterial preparations are used for seed treatment, and if necessary, spraying is performed when the first symptoms appear. The fungal preparation Trichodermin is applied into the planting hole 2-3 days before planting the transplants in an amount of at least 10 billion spores per plant. The preparation is applied again in the form of a suspension, which is watered under the root at the same application rate. For example, the use of Trichoderma strains TK-13 and Istoksky reduced infestation by 80-90 % and increased the yield by 3.3 kg/m². The application rate is 150 billion spores/m². It is possible to use a preparation based on fungi of the genus Gliocladium.

Chemical and biological protection methods: from disinfection to stimulation

Protection of cucumber against root rots begins long before planting transplants. The cleanliness of the substrate and planting material is the main barrier to infection at the start of the season. For this purpose, mandatory disinfection of equipment and tillage with sterilizers are carried out.

1. Soak containers, pots, and trays in disinfectant solutions when preparing for a new crop rotation.
2. Treat the soil or substrates with sterilizers: Basamid Granulat, methyl bromide, or Vydate.
3. Perform seed treatment using the semi-dry method with TMTD (thiram) 3–4 weeks before sowing.

To increase the non-specific immunity of plants to root rots, hormonal preparations are used: Immunocytophyte, Narcissus, EL-1, humates, and Crezacin. These are used to treat seed when preparing for sowing or to spray on growing plants. Preparations based on metalaxyl, oxadixyl, and their analogs are most active against Pythium and Phytophthora. Benzimidazole, carbamate, morpholine, and acylalanine fungicides cope with Rhizoctonia rot.

When planting cucumber in a recently steamed substrate, non-pathogenic root rot may occur. Thermophilic bacteria and fungi developing in it cause temporary soil toxicity, which leads to the dying off of root hairs. To quickly restore soil health and its fungistatic properties, biological preparations are used: Bactofit, Planriz, Trichodermin, or Pseudobacterin. Applying high-quality compost immediately after steaming or sterilization also helps.

Why cucumber wilts: infectious and physiological causes

Cucumber wilting in greenhouses is a common problem that can completely destroy plantings. Often, this happens due to the infection of the plant's vascular system by pathogenic fungi. However, wilting can also be linked to simple root underdevelopment in excessively dense soil. In this case, an outwardly healthy root system is simply unable to provide the plant with water and nutrients.

Another frequent cause is the penetration of a fungal pathogen into plant vessels due to errors in forming ridges. On narrow and high ridges with sloping edges, irrigation water quickly runs off into the furrows without reaching the roots. If the soil temperature rises to 28 °C during this, the roots weaken and open the path for infection. The disease manifests as marginal spotting of the lower leaves, necrosis of tissues between veins, and loss of turgor in the upper part of the bush.

The destructiveness of wilting is extremely high, especially in plastic greenhouses without regular soil disinfection. Infection of the aerial part by Fusarium can destroy more than 50% of greenhouse plants, with wilting foci expanding every year. Initial symptoms in the form of reddening and rotting of roots can appear as early as the transplant stage and continue until the end of the season. A similar outbreak of root rot is often observed immediately before the end of the growing season.

• Seed treatment time with TMTD — 3–4 weeks before sowing
• Application rate of TMTD — 4–8 g per 1 kg of seed
• Toxicosis period after soil steaming — up to 1 month
• Critical soil temperature for infection — 28 °C
• Plant loss due to Fusarium infection — more than 50%

To prevent wilting and restore soil health, follow these agrotechnical rules:

• Strictly observe the technology of substrate preparation and their expiration dates, prioritizing modern low-volume technologies.
• Create optimal microclimatic conditions for cucumber growth to prolong the growing season and fruiting.
• Systematically apply decomposed compost to the soil surface to stimulate the formation of additional roots.
• Periodically pierce the soil with a pitchfork to improve aeration of the root zone.
• Monitor the drainage of the nutrient solution during drip irrigation, preventing it from stagnating in trays or mats.
• Form plants correctly and harvest in a timely manner to reduce the overall load and slow down the aging of the root system.

Stem and root forms of Fusarium: symptoms and biology

Fusarium wilt of cucumber manifests differently depending on the stage of the growing season and the form of the disease. Tracheomycotic clogging of vessels, leading to plant death due to disturbed water exchange, is relatively rare. More often, symptoms change after the start of mass harvesting of cucumbers, especially on sunny spring days. The development of the root system and stem vessels remains normal, but a silvery-white Fus. oxysporum mycelium appears at the base of the stems. The disease is widespread at this time.

For accurate diagnosis, the pathogen is inoculated onto an agar medium. After a few days, a well-developed white or pinkish mycelium with characteristic conidia grows on it, and microsclerotia are formed in older cultures. Without competition from other microorganisms, even single viable fungal conidia in the soil form large colonies. The substrate in this area becomes unsuitable for plant cultivation without additional disinfection and the application of biological products.

Disease development is directly related to the quality of crop care. The mere presence of a pathogen in a plant does not necessarily lead to its wilting. With optimal cultivation technology, Fusarium affects plants significantly less often than under poor management conditions. The main source of infection in greenhouses is contaminated seed, and to a lesser extent, soil and crop residues.

• Reduction in disease incidence with proper care — 3-fold
• Growth of diagnostic colony on agar — 4–5 days

Protection methods: seed preparation, substrate disinfection, and chemical treatments

The main challenge in combating the stem form of Fusarium is its high resistance to fungicides. Due to regular treatments in greenhouses, the pathogen rapidly develops resistance to preparations. For this reason, standard chemical seed treatment against latent infection is ineffective: TMTD showed weak activity in experiments, and Fundazol showed satisfactory activity. Reliable elimination of latent Fusarium infection is only possible through stepwise pre-sowing seed heating.

1. Heating dry seeds for 1 day at a temperature of 35 °C.
2. Holding seeds for 3 days at a temperature of 55 °C.
3. Final heating for 1 day at a temperature of 70–72 °C.

Prevention of the root form of Fusarium is based on the mandatory preparation of greenhouse substrates, transplant mixtures, and composts. They are disinfected with steam or methyl bromide, followed by the mandatory application of biological products. This form is characterized by a low level of resistance to chemicals, so chemical protection does not cause problems when the disease spreads. The working solution of the fungicide is poured under the root of the affected plant, and in case of focal disease development, under all plants in the developing focus.

A few days after soil disinfection and application of biological products, it is advisable to conduct a microbiological analysis of the substrate. If pathogenic fungi are detected, their resistance spectrum to fungicides is determined in the laboratory. For protection against the root form, treatments with Bayleton or permitted benzimidazoles are usually sufficient. Combatting the stem form requires individual selection of preparations based on isolate analysis, as most strains from seeds and stems are resistant to TMTD, Fundazol, Saprol, and Euparen.

• Protective action period of fungicides — 2–3 weeks
• Interval before substrate analysis after disinfection — 5–6 days
• Frequency of fungicide drenching under the root — 1–2 treatments

Rhizoctonia: biology of the pathogen and control measures

Rhizoctonia affects almost all organs of the cucumber, with the exception of the flowers. The disease is most dangerous in spring plastic greenhouses, where the loss of transplants can reach catastrophic proportions. The causative agent of the disease is the fungus Rhizoctonia solani (basidial stage — Pellicularia filamentosa).

Symptoms of Rhizoctonia appear at different stages of crop development:

• On transplants: in the root collar area, the tissue yellows, dries out, and dies, causing the seedling to perish. Small, rounded or oval yellowish-orange spots appear on the cotyledon leaves.
• During the fruiting period: elongated depressed light-brown ulcers up to 2.5 cm long form on the petioles, and large, diffuse brown spots form on the leaves. On the top and underside of fruits in contact with the soil, dry, depressed brown spots with a dark border appear, which may coalesce.
• During fruit storage: in case of long-term storage, sporulation develops on the cucumbers in the form of velvety gray-black cushions.

The pathogen is extremely hardy, has no dormant period, and actively develops throughout the entire arable horizon of greenhouses. On the plant, the parasite lives exclusively in the form of mycelium, without pseudosclerotia or basidial sporulation. The young sterile mycelium is colorless or pinkish, branches at right angles, and has rare septa. On artificial agar medium, the fungus forms brown mycelium without signs of sporulation.

• Pathogen detection depth — more than 25 cm
• Temperature range for development — from 3 to 25 °C
• Soil moisture for development — from 40 to 100% of field capacity
• Substrate acidity (pH) — from 4.5 to 8
• Infection persistence in soil — 5–6 years

Controlling rhizoctonia requires a complex of measures aimed at improving the substrate and protecting growing crops. It is important to strictly monitor air temperature and soil moisture, increasing the cucumber's natural resistance to the pathogen. The protection schedule includes biological and chemical preparations.

1. Seed treatment before sowing. Use bacterial preparations such as Pseudobacterin-2, Bacto-fit, or Planriz. During seed germination, a protective coating of beneficial microorganisms forms around the root.
2. Application of fungal biological preparations. To suppress infection in the substrate, use Trichodermin and Gliocladin.
3. Preventive spraying. During the growing season, conduct protective treatments with Quadris. In case of a risk of damage to leaves and fruits touching the ground, use Bacto-fit.
4. Curative treatments at the first symptoms. Spray the planting with a 0.2% working solution of preparations based on mancozeb or copper oxychloride: Dithane M-45, Oxichom, or Ridomil MC.

Ascochyta blight: diagnosis of stem and fruit forms

Ascochyta blight (causative agent — Ascochyta cucumis, teleomorph stage — Mycosphaerella melonis) affects cucumber mainly during the fruiting period; it is recorded much less frequently on transplants. A new form of the disease that causes browning of the fruit flesh is particularly dangerous. Due to this defect, the volume of non-standard produce in greenhouses increases to 37–50%. Symptoms of the disease depend on the site of infection entry.

Symptoms of the infection are distributed across the above-ground organs of the plant:

• On stems (with early infection): round or oval watery grey-green spots appear. They rapidly expand, turn brown, and whiten upon drying, girdling the stem. The epidermis cracks with the release of milky or brown exudate, and fiber maceration occurs.
• In nodes and on stubs: infection foci are often concentrated in stem nodes and on long stubs left after pruning leaves and shoots. Affected areas are densely covered with black dots of pycnidia.

Characteristic signs of Ascochyta blight often manifest in stem nodes and on long stubs left after the removal of shoots and leaves. All affected tissues are abundantly covered with numerous black dots — pycnidia. Recently, the simultaneous development of the pycnidial and teleomorph stages of the pathogen has been noted on plants.

Leaves are affected during the cucumber's fruiting period. The disease usually starts from the edge of the leaf blade. Large, 4–5 cm in diameter, diffuse spots with a chlorotic zone at the periphery form in the affected areas, sometimes covering half the leaf. The leaf tissue in the spot zone first becomes brown, later — whitish, and is densely covered with pycnidia arranged randomly or in concentric rows. The affected tissue dries out and crumbles. Sometimes two spots develop on a leaf. This causes rapid death of the entire leaf blade. With diffuse spread of the fungal mycelium, leaves become chlorotic, then turn yellow, redden, lose turgor, and wilt rapidly. On fruits, Ascochyta blight manifests in two forms. The first form — the disease starts from the base or the top of the fruit. The affected tissue dries out slightly, appearing as if cooked, but maintains a solid structure and is quickly covered with pycnidia. Subsequently, the entire fruit turns black and dries up (mummifies) or decomposes with wet rot. Cracks or ulcers from which gum is released often form on the surface of seed-bearing fruits. This phenomenon is often observed during their ripening period. The second form is characterized by the appearance on the green fruit of small, dry ulcers, 3 to 5 mm in diameter, sunken into the tissue and abundantly covered with pycnidia.

In recent years, a new form of the disease has appeared: browning of the fruit flesh. The first symptoms manifest as whitening of the upper part of the fruit; later, a rust-colored spot is visible inside the fruit, which eventually becomes slimy. Secondary bacterial rot begins to develop, which gradually encompasses the entire fruit. Fig. 82. SYMPTOMS OF ASCOCHYTA BLIGHT ON A LOWER CUCUMBER LEAF \ = № |

4 ia \ / \ zhi —-* Biology of the pathogen. Light-brown pycnidia usually form on the stem, the upper side of the leaves, less often — on other parts of the plant. They are immersed, semi-immersed, or located on the surface of infected organs. They have a spherical-flattened shape with a diameter of 100–200 µm. The pycnidium opens with a round orifice (pore) 20 µm in diameter, surrounded by small dark cells. The pycnidia wall on leaves is thin, while it is thick on other parts of the plant. Conidia are cylindrical, some slightly club-shaped or elongated-ellipsoidal, not constricted 11–20 x 2.5–4 µm. The causative agent of Ascochyta blight does not multiply in the soil and is rarely isolated from greenhouse soils. Research has shown that the pathogen is present in cucumber seeds in a state of anabiosis. The fungus is activated in the phase of initial formation of true leaves. At first, dimorphic mycelium, which looks little like the Ascochyta blight agent, is released from the xylem of the crown, and only after the formation of the third leaf does fully developed mycelium grow. The pathogen intensively colonizes the crown tissues, then moves upward through the vessels. In the leaves of the lower tier, it releases phytotoxins, thereby causing the appearance of chlorotic or reddish spots on them. The fungal generation cycle on the lower leaves ends in March–April with the formation of sporulating pycnidia and perithecia on petioles in early May. Aerogenic re-infection of plants begins. The diffuse spread of the pathogen up the stem accelerates, and soon the leaves of the middle tier must also be removed. Therefore, it is advisable to start cutting the lower leaves only after signs of ripening (blackening) of the pycnidia appear. Such a practice curbs the acropetal spread of the mycelium. In June–July, cucumber Ascochyta blight becomes a common occurrence Fig. 83. SYMPTOMS OF FRUIT FLESH BROWNING AS A FORM OF ASCOCHYTA BLIGHT MANIFESTATION. E By the end of the growing season, the endogenous mycelium of the fungus penetrates the upper tier of leaves and the fruits. During the storage of such green fruits, wet brown spots appear on them, gradually becoming covered with the fungus's pycnidia. The opinion that fruits are infected with Ascochyta blight during storage in a warehouse is incorrect. The pathogen does not spread into roots or soil. The lower part of the stem is usually affected by sporulating colonies of the pathogen; on some stems, such colonies also form in the upper tiers. The fungus is capable of developing at a temperature of 10–32° and in a wide range of relative air humidity 20–100%. The disease acquires an epiphytotic character in April. At this time, solar activity is high, but the outdoor temperature does not yet allow for the full use of transom ventilation. Favorable conditions for the phytopathogen (high relative air humidity and increased temperature) are created in the greenhouse. SPORULATION OF Ascochyta cucumis ON A CUCUMBER PETIOLE. “ya 1 l r ya ” \ y%.