Combating bacterial wilt of tomatoes and potatoes in greenhouses

Bacterial wilt of tomato and potato: symptoms and containment measures

Bacterial wilt is an extremely dangerous quarantine disease capable of completely destroying potato and tomato crops. Its causative agent is the bacterium P5eидотопа$ (Каюта) зо]апасеагит (Е.Е.5т). In tropical and subtropical regions, race 3 of this pathogen is widespread, but outbreaks have also been noted in Europe and Russia. In a greenhouse, the infection spreads rapidly, carried by irrigation water droplets and during plant care.

The disease can occur in two forms. In the acute form, bushes wilt and die rapidly without prior leaf yellowing or stunted growth. The chronic course manifests more slowly: adventitious roots form on the basal part of the stem, leaves turn yellow, and fruits become smaller and drop. On a cross-section of the stem, brownish-yellow rings of vessels clogged with slime are clearly visible, and on the fruits, blossom-end rot develops in the form of a dense brown ball. For accurate identification of the pathogen, the oxidase and urease test, the indirect immunofluorescent staining method, and a pathogenicity test on tomatoes are used.

• Latent period of the disease — more than 2 months
• Diameter of the brown formation in the fruit — 1–3 cm
• Concentration of Phytolavin-300 for soil drenching — 0.6–1.0 %
• Consumption of the working solution per hole — 200 ml
• Radius of the protective zone around the outbreak — 7–10 m
• Concentration of liquid glass in the mixture — 0.15 %

To prevent the spread of infection in the greenhouse, it is necessary to strictly observe quarantine measures and act immediately upon detecting the first diseased plants. Infected seeds can also become a source of contamination. Protective measures are carried out according to a clear scheme.

1. Completely remove plants affected by wilt from the greenhouse.
2. Drench the soil under neighboring healthy bushes within a radius of 7–10 meters with a working solution of 0.6–1.0 % Phytolavin-300, using 200 ml for each hole.
3. Spray the plantings with a 0.6–1.0 % Phytolavin-300 solution with the addition of 0.15 % liquid glass to create a protective film that hinders the transfer of infection for 2–3 weeks.

Bacterial stem rot: diagnostics and developmental characteristics

The causative agent of bacterial stem rot is the pathogen C! amBaсеr nисdаnеnt$15 зизр. nис 9dа-nеn$5 (Эт) Па\5 её а|. [syn.: Согуперамейит тсШадапеп$Е]. At the seedling stage, the disease practically does not manifest itself; the first symptoms usually become noticeable only by the beginning of fruiting. The infection systemically affects the vascular system of plants, blocking mainly the sieve tubes of the phloem. Primarily, however, bacteria penetrate through the spiral vessels of the xylem.

A typical sign of bacterial rot is the darkening of the vascular ring, which is clearly visible on a cross-section of the stem and at the base of the leaf petioles. The disease begins with unilateral wilting of the lower leaf leaflets, their edges turn yellow and curl. Over time, the leaves turn brown and dry out but remain hanging on the plant. Dark stripes appear on the stems, the epidermis ruptures, and bacterial exudate is released through the cracks, which is carried around the greenhouse by insects, water splashes, and the hands of workers during pruning. Deep brown ulcers form on young sepals, petioles, and fruit stalks, and the pith of the stem turns yellow, dies, and forms cavities.

Flat spots no larger than 3 mm appear on the fruits, snow-white on the periphery and dark in the center. On green fruits, the spots are white, located mainly around the fruit stalk. On ripening fruits, the spots turn yellow, and small black cracks appear in their center, resembling a bird's eye, which is why this form of the disease is called "bird's eye rot".

Another, more common and more dangerous form of fruit damage is the internal one, caused by the penetration of bacteria into the fruits directly from the affected vessels. The ulcers that formed on the fruit stalks gradually deepen due to secondary spread of the infection to such an extent that the bacteria reach the vascular system and enter the fruit through it. With early infection, such fruits usually have an ugly shape, and the seeds inside are underdeveloped and non-viable. Many of these fruits drop. With later infection, the fruits resemble healthy ones in appearance, but the vascular strands leading to the seed chambers in such fruits turn yellow. Seeds obtained from such fruits carry the infection, but their germination remains quite high. Bacteria are located on the surface of the seeds or accumulate between the seed coat and the endosperm. The consistency and taste of the fruit deteriorate.

Often, disease symptoms are not noticeable on infected fruits, and seeds obtained from them contain the pathogen.

Pathogen Biology. Aerobic, non-spore-forming, Gram-positive, non-motile rods, 0.3-0.4 x 0.8-1.0 µm in size. Bacteria penetrate plants most often through mechanical injuries. Via phloem vessels, the bacteria reach other organs, including fruits, causing internal infection. Cultural properties. On meat-peptone agar, colonies are round, smooth, and raised, initially colorless (almost transparent), and later cream-yellow. When glucose is added to the medium, colony growth accelerates, and they become larger and pale yellow in color. On a potato tuber slice, colonies grow rapidly, with a bright yellow color. Broth becomes turbid slowly, and after 3-4 days, a faint mucous sediment appears. The bacteria are resistant to drying. The optimal temperature for growth is 25-27°, growth ceases at 47°, and bacteria die at 50-53°.

High temperature and humidity promote disease development. Overhead irrigation of plants in hot weather and even spraying with pesticides can lead to an epiphytotic.

The pathogen is capable of infecting only plants from the Solanaceae family. Practically all tomato cultivars and hybrids cultivated in both protected and open ground are highly susceptible to bacterial canker. Increased resistance is shown by the open-ground cultivars Rakoustoychiviy 12 and Rakoustoychiviy 69, obtained by crossing wild currant tomato with commercial cultivars.

Sources of primary infection. The source of primary infection of bacterial canker is infected seed. It was precisely with these that the pathogen was first introduced into the territory of the former USSR in 1936. Other sources of primary infection include contaminated plant debris and soil, which is often observed in continuous tomato cropping in small-volume culture. It has been established that bacteria in the soil lose their viability within 30-40 days. However, in air-dry soil and in plant debris, bacteria can persist for more than eight months, including in frozen soil.

Spread occurs under conditions of high air humidity when plants are injured (during side-shoot removal, leaf pruning, or fruit harvesting). It has been established that bacteria are capable of penetrating neighboring plants through leaf hairs, which is quite possible in dense plantings.

Diagnostics. Early detection of infection in seeds is of great importance in combating this disease.

The simplest method is Gram staining of sections or imprint smears. This feature is quite accurate because, among wilting pathogens, this is the only Gram-positive bacterial species. To detect the pathogen, enzyme-linked immunosorbent assay (ELISA), immunofluorescence method, and polymerase chain reaction (PCR) are also used.

Protection measures. Agrotechnical practices. Resistant hybrids are unknown. To protect plantings from bacteriosis, seed is obtained only from healthy plants and fermented in its own juice without adding water for 96 hours.

It is important to disinfect the soil and use well-decomposed compost for filling hotbeds, which is a guarantee of a good harvest.

The greenhouse is frequently ventilated. Overhead irrigation is stopped upon the appearance of the first symptoms. The concentration of the nutrient solution is reduced, and the acidity of the substrate or solution is increased, as this inhibits pathogen development. Diseased plants are removed from the greenhouse and destroyed. Crop rotation with the return of tomatoes to the same spot no sooner than after a year allows for the elimination of the soil source of infection.

To limit secondary infection, before scheduled side-shoot removal, phytosanitary cleaning of the plantings from diseased plants is performed (at least twice a month). To reduce the spread of bacteriosis throughout the greenhouse, work with diseased plants should be started after tending to healthy ones. For removing side shoots and leaves, it is desirable to use a special tool rather than breaking them off by hand.

Biological agents. To reduce the pathogen pool, seeds are soaked in a 0.2% working solution of Phytolavin-300 for two hours. Subsequently, transplants are sprayed twice, starting from the 1-3 true leaf stage, with a 0.2% suspension of Phytolavin-300. Good results are obtained by dipping the roots of transplants into the same suspension. During the period of intensive fruiting, it is recommended to spray plants with Planriz.

Previously, it was suggested to use medical antibiotics (streptomycin, penicillin, aureomycin), but now their use is prohibited in plant production. Good results are obtained using products based on humates.

Pathogen — Pseudomоnas corrugata Kaper et Sacket.

Harmfulness. It causes wilting and death of plants, most intensively observed at the very beginning of fruiting. The greatest harvest losses are possible with early infection through seeds. In this case, a significant part of the plants dies. Symptoms. The first signs of the disease appear primarily on well-developed tall plants during the formation of the first clusters. Elongated, slightly depressed, brown-colored spots appear on the stems, located not far above the substrate, which subsequently crack. A mass of bacteria accumulates in the vessels, which flows out in the form of a creamy-white exudate from cracks and wounds.

On a longitudinal section of the affected stems, necrosis of the pith is visible, as well as of adjacent tissues (vessels, bark, epidermis). If we consider the process of necrotization in dynamics, it is first noticeable that the pith tissues seem to become saturated with water, become glassy, then darken and gradually dry out. On an outwardly healthy stem, in the basal part, and sometimes higher, a multitude of densely located adventitious (aerial) roots appear. The leaves, mostly starting from the top, lose turgor, darken, and wilt, acquiring a scalded appearance. In some cases, chlorosis and wilting of the upper leaves are observed.

A network of light veins appears on green fruits, which persists on mature fruits; when they are opened, necrosis of the seed coat can often be seen. Shaking the plants leads to fruit drop.

Cultural characteristics. When bacteria are sown on potato agar, raised, round colonies of cream color with a diameter of up to 1.5 mm with an uneven surface and a slightly wavy edge form after two days (Saisine, Nesreg, 1978). On glucose-yeast agar, the colonies are flat, yellowish-green in color, and also have an uneven surface. Bacteria release a yellowish-green non-fluorescent pigment into the medium.

Temperature conditions for colony growth:

• Growth cessation: 41°
• Optimal values: from 26 to 28°

Primary infection of plants occurs mainly at early stages of development, as the source of infection is, as a rule, seed, in which the pathogen can be localized not only in the coat but also in the seed embryo.

The pathogen spreads systemically throughout the plant, penetrating all organs, but the most favorable conditions for its development are created in the pith parenchyma of the stem. Over time, bacteria spread to adjacent tissues, causing bark and epidermis necrosis, which look like elongated spots.