Diagnosis and prevention of root rots in tomato seedlings in greenhouses

Symptoms of these diseases appear within six weeks of transplanting. Affected plants often cease development or, soon after rooting, become dark green and wilt, especially under stress conditions, such as high temperatures, or as a result of waterlogging or drying out of the soil. The lesion on the root collar at the soil surface or slightly below can partially or completely girdle the entire stem.

Greenhouse soils are often infested with one or another pathogen of root rot. In cold, wet soils, Phytophthora species are most common, while in well-drained ones, Rhizoctonia occurs. Sometimes root rot is caused by Botrytis cinerea, especially if seedlings lag in development during the growing period and cotyledons wither, or if wilting is the result of planting too deep, where cotyledons are partially or completely covered by soil. A very dangerous pathogen in some greenhouses is the fungus Pythium ultimum — mainly a soil-borne pathogen. Initial infection by this pathogen develops on plant tissues located in the soil.

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Fig. 7.1. Phytophthora root rot. The affected area is located right at the soil surface.

It is not easy to distinguish the symptoms of various root or collar rots. Phytophthora species typically cause a dark brown (almost black) lesion at the soil level, which often extends along the root collar deeper into the soil, to the root itself. The roots always turn slightly brown. When Rhizoctonia solani is involved, dry spots develop at the base of the stem that do not involve the tissues of the main root. The affected area is often light brown, and light brown mycelium, visible under a magnifying glass, sometimes appears on its surface. Botrytis cinerea forms light brown spots, often located around cotyledon scars. In a humid environment, the pathogen actively sporulates throughout the stem, forming a dense layer of conidiophores and spores.

The affected area is usually located at or slightly above the soil level, however, external signs of the disease are absent. With a strong magnifying glass, brown pycnidia can be detected on the diseased tissues, which are easily confused with lighter heads of glandular hairs of the same size on stems of tomato. Examining the stem above the lesion area will help distinguish the pubescence from the fungus pycnidia.

With age, the resistance of tomato to Phytophthora and Rhizoctonia increases, and 6 weeks after transplanting they are rarely affected by root rots. Often, when symptoms are detected, the disease is already in full swing, but the remaining healthy plants have by this time reached an age at which they have formed a relative degree of resistance.

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Fig. 7.2. Botrytis cinerea sporulating on the affected area of a tomato stem after two days of incubation in a warm and humid atmosphere. plants at this time have reached an age at which they have developed a relative degree of resistance.

Sources of infection for root or collar rots are the same as for damping-off.

Control. Soil from under the seedlings must be disinfected, or a carefully prepared, soil-free compost should be used. Greenhouse soil also requires disinfection. If plants with signs of root or collar rot are found, regardless of the pathogen, the development of adventitious roots above the affected stem area can be stimulated by mulching the plants with moist peat or disinfected soil.

In controlling Phytophthora collar rot, good results are obtained by incorporating etridiazole into the compost or by drenching planted crops with a dithiocarbamate fungicide solution, for example, a zineb solution.

Against Rhizoctonia solani, quintozene is used by incorporating it into compost or by dusting the soil.

To prevent the appearance of B. cinerea, it is necessary to remove withered cotyledons before transplanting to a permanent location and to avoid planting too deep. If the disease does appear, targeted (at the base of the stems) high-volume spraying with iprodione or vinclozolin will curb further spread of the affected area.

Irrigation with a captan solution shortly after transplanting into the soil and a second time three weeks later ensures the suppression of root rot caused by Pythium.

Fig. 7.3. Severe wilting of a tomato plant as a result of root rot infection. <ТУЯ, КОРНЕВЫЕ ГНИЛИ < 247

Root rots appear during the process of seedling cultivation or at any time of the season after transplanting into the ground. Usually, root rot infection manifests as stunted growth, the appearance of nutrient deficiency symptoms on leaves (especially interveinal chlorosis), and dark green coloration of young plants. As root rot progresses, symptoms become more pronounced, and plants wilt. Young leaves and growing shoots are affected first, wilting for a short time during the day, especially at high temperatures in the greenhouse. A return to normal turgor often occurs in the evening and at night, but sometimes wilting persists.

tomato verticillium or fusarium wilt, although in the latter case, the wilting begins with the lower leaves. With vessels, but if with root rot it covers a section of the stem longer than, soil, then with vascular wilt it

The first symptoms of root rots usually appear on plants in the outer rows of a planting, at the ends of the greenhouse, and under the gutters, as these areas are particularly difficult to disinfect.

Root rots, caused by various fungi, differ to some extent in their symptoms and, consequently, in the control measures that can be applied against the pathogens.

Brown root rot and corky root (Pyrenochaeta lycopersici)

This disease develops in almost all greenhouses where tomatoes are grown. The pathogen also affects plants closely related to the tomato and even survives on the surface of roots of plants from other families, for example, lettuce, and possibly some weeds.

The fungus grows very slowly and therefore cannot quickly colonize the soil or the root system. Even in cases where the concentration of inoculum in the soil is sufficiently high, the first symptoms of infection on the roots appear several weeks after transplanting into contaminated soil. Signs of the disease appearing on the crop can be detected only after the formation of the first truss begins and the others are set.

With thorough soil disinfection before planting transplants, symptoms of rot infection sometimes appear only at the end of the season, and sometimes they cannot be detected at all. However, when digging up the roots, mild infection is discovered, even though the crop harvest was quite satisfactory.

The first symptoms are light brown spots up to 0.5 cm long on thin rootlets, which are primary infection sites. Their early appearance (16 weeks after planting) and large numbers indicate a very high concentration of inoculum in the soil. Harvest losses are proportional to the degree of root rot infection in plants during this period.

On larger roots, dark brown, corky, slightly raised spots appear. Corky tissues develop only on large roots, therefore in plants grown in peat compost, where the development of a mass of thin rootlets is stimulated, symptoms of corking occur very rarely, although brown rot infection can be very severe.

Microscopic examination of affected tissues reveals the mycelium of the fungus, which has penetrated the host cells and formed microsclerotia on the outer tissues of the roots. These microsclerotia are quite resistant to unfavorable environmental conditions and survive in the soil between two crops. After the plants are removed from the greenhouse, the outer root tissues remain in the soil, and the dormant structures of the pathogen are released during the decomposition of plant residues, contaminating the soil.

Often, corking spreads to the base of the stems of severely affected plants, but, as a rule, only for a few centimeters. The affected areas have a diffuse shape, which makes it easy to distinguish corking from crown rot.

The slow growth of the pathogen in the soil can be considered an advantage if one plants transplants with large clods of disinfected compost. From this point of view, early-maturing cultivars whose transplants are grown in large pots will be protected for longer against severe brown root rot, unlike plants grown from transplants in small pots.

Fig. 7.4. Brown root rot and corky root: a — root system with areas of browning and rot on small roots (1) and corking of large roots (2); b — zones of corking on large roots. Phytophthora root rot (various Phytophthora species)

A widespread root rot caused by these pathogens usually develops soon after planting the transplants into the soil and appears much more frequently during the early phases of crop development than other root rots. Light brown spots of rot are visible on the affected young roots, but the disease often begins from the taproot. Corking does not occur, although, on the surface, the browning caused by Phytophthora is difficult to distinguish from that caused by Pyrenochaeta lycopersici. The disease mainly affects tomatoes grown in partially heated or unheated greenhouses and is encountered much less frequently in early tomatoes in heated greenhouses.

This fungus is considered a pathogen of root rot, but, as a rule, it simply colonizes old, already decaying roots. Penetrating damaged outer root tissues, C. coccodes forms black structures, from which the disease derives its name, black dot (anthracnose). The outer root tissues with symptoms of black dot have usually already sloughed off from the central vascular bundles. Calyptella root rot (Calyptella campanula)

Root rot, caused by the basidiomycete Calyptella campanula, has recently been described in England. Infected plants wilt during the ripening period of the first fruit cluster. A diffuse light-brown rot develops on the roots of diseased plants without thickening or swelling of the tissues. The vascular tissues of the stems at the root collar sometimes turn brown. The most typical sign of the disease is the development of lemon-yellow fruiting bodies of the pathogen, resembling toadstool caps. Such structures, 0.5–1.0 cm in diameter, are located on the soil surface near tomato stems. Sometimes they fuse with rotting roots. The fruiting bodies develop in early or mid-summer, but they can usually be detected no earlier than August.

Root rots of tomato seedlings in greenhouses are a dangerous problem that can deprive farms of a significant portion of their yield. To combat them successfully, it is important to accurately identify the pathogen, since methods effective against some pathogens may prove useless or even harmful when infected by others.

The fungus Colletotrichum coccodes (referred to as C. campanula in the source) was until recently detected only on tomatoes grown in soil without transplanting, but now it is also encountered on plants in peat bags. The development of this pathogen is favored by waterlogged soil, regular irrigation, and drip irrigation.

Another pathogen — Spongospora subterranea (the causative agent of powdery scab of potatoes) — sometimes infects tomato roots if they are grown without soil disinfection in areas where potatoes were grown in previous years. The disease is easy to recognize by swellings on the roots, similar to root-knot nematode galls, but usually smaller in size. The disease is rare and is easily eliminated by thorough soil disinfection.

• The main method of soil disinfection is steaming
• Chemical disinfection agents — methyl bromide, dazomet
• Regular irrigation agent for brown rot — zineb solution
• Seedling compost component against late blight — etridiazole

Protection methods: disinfection, agrotechnics, and resistant hybrids

Thermal and chemical treatment methods are used to control most root rots. If steaming is impossible and chemical disinfection does not provide the desired effect, the crop is isolated from the soil. Plants are grown in pots on the soil surface, or alternative substrates such as peat and straw are used. This reliably protects tomatoes from soil-borne pathogens, although it complicates nutrient management and increases labor costs.

1. Perform thermal disinfection (steaming) of the soil or treat it with methyl bromide or dazomet.
2. If disinfection is not possible, isolate plants from the soil using pots, peat bags, or straw.
3. At the first signs of wilting, stimulate the formation of adventitious roots by mulching the root collar.
4. Conduct regular irrigations with a zineb solution to prevent brown root rot and corky root rot.
5. Incorporate etridiazole into the seedling compost to suppress Phytophthora root rot.

A reliable way to protect against brown root rot and corky root rot (causative agent — Pyrenochaeta lycopersici) is the use of resistant cultivars and rootstocks. They also possess high resistance to Verticillium and Fusarium wilt, but remain susceptible to Phytophthora root and stem rot. Seed material should be selected taking their genetic profile into account.

* Possesses additional resistance to root-knot nematodes.