Diagnosis and control measures for bacterial diseases of tomatoes

And Fig. 7.13. Bacterial pith necrosis of the stem in longitudinal section; arrows indicate necrosis in the pith cavity.

adjacent to the vascular bundles, it usually spreads down the stem to its base, but apparently does not involve the roots. Furthermore, the disease does not spread to the fruits, although the core of the fruit stalk may also be affected.

Diseased plants do not necessarily die and can produce a satisfactory harvest. Sometimes the only sign of infection is a discoloration of the stem pith, noticeable only when leaves are removed. Creamy-white bacterial slime sometimes exudes from leaf scars.

Control. Since there is a close correlation between the growth habit of tomatoes and the appearance of pith necrosis, one can try to regulate crop development by changing the nutrient regime. As a rule, a high potassium content in the basal fertilizer and liquid top dressing, which also contains a lot of potassium, will help curb overactive crop growth. Spraying with fungicides against pith necrosis is ineffective.

Bacterial soft rot (causal agent Erwinia carotovora var. carotovora)

Sometimes soft rot of stems and fruits develops on tomato, although it rarely affects a large number of plants in a single crop at once. Symptoms on the stems are dark brown or black softened water-soaked spots, usually located in the lower part of the stems. If the infection develops near the fruit cluster, it can spread to the fruit stalks and fruits. The rot progresses rapidly; the affected plant wilts and dies, especially with vigorous vegetative growth of the crop and high temperatures in the greenhouse. The pathogen penetrates plants through wounds. It is rarely transmitted mechanically, but can move from plant to plant through touching side shoots and through the scars of fallen leaves.

Control. Bacterial soft rot does not always require control measures. As a rule, with proper plant care that ensures uniform development, the disease does not pose a serious problem. If soft rot does appear, diseased plants should be removed from the planting and hygiene standards should be carefully observed. Bacterial spot (Pseudomonas syringae pv. tomato)

Bacterial spot occurs on the Channel Islands, in countries of continental Europe, and very rarely in Great Britain. Very small (2–3 mm in diameter) dark brown spots, often with a yellow halo, develop on the leaves of affected plants. Sometimes the spots merge, and the leaf dies. The same spotting can develop on the stems, sometimes pro-

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Control. Affected leaves must be removed. Overhead irrigation should be avoided and the relative humidity of the air should be kept at a minimum level. To some extent, bacterial spot is suppressed by copper-containing fungicides when applied via high-volume spraying.

This disease is very common on tomato in dry climate conditions. It appears in greenhouses in Southern Europe, but has not yet been detected in more northern regions. When infected with powdery mildew, yellow spotting develops on the upper side of the leaves, and a white to light-gray coating of the pathogen's mycelium develops on the underside, which externally resembles an infection of leaf mold.

Epiphytotics of powdery mildew develop under the following conditions:

• low relative humidity of the air;
• temperature range: 15—25 °С.

With insufficient irrigation, tomato is affected particularly severely, while with overhead irrigation, the disease rarely causes serious damage.

Pepper, eggplant and cucumber are also hosts of the pathogen. It is easily spread when spores are released from conidia into the air.

Control. Proper greenhouse management, especially with an overhead irrigation system, provides a certain degree of control over the development of powdery mildew. If the disease has already developed on greenhouse tomato, chemical control does not produce the desired result, although some success has been achieved with benzimidazoles. Chlorothalonil is effective to a certain extent against L. taurica.

There have been no reports of bacterial wilt affecting greenhouse tomatoes in Europe; however, this disease poses a serious problem in some countries with warm climates. In addition to tomatoes, other species of the Solanaceae family act as hosts for the bacteria, in particular potatoes.

Typical symptoms of bacterial wilt include the rapid wilting of the entire plant, which, however, is not accompanied by yellowing of the leaves or stunted growth. A cross-section of the stem, taken near the soil surface, reveals a darkened, water-soaked pith; when pressed, a grayish slimy exudate oozes from the stem. In later stages of disease development, the pith decays and the stem becomes hollow.

The pathogen resides in the soil and infects plants through the roots and the base of the stem. The spread of infection from plant to plant occurs during crop maintenance and via water splashes. Signs of wilting appear as a result of the bacteria blocking the vascular system. None of the industrial cultivars possess resistance to P. solanacearum, although breeders have discovered some sources of tolerance.

Control. Affected plants should be removed from the greenhouse immediately, and overhead irrigation and other methods of humidification should be discontinued. Chemical methods for controlling bacterial wilt have not been developed.

White leaf spot, or septoria leaf spot (Septoria lycopersici). This disease appears in Great Britain only occasionally, but it represents a major danger in many parts of the world, particularly in North America and Africa. Septoria epidemics are a common phenomenon when growing tomatoes in open soil in countries with sufficiently high summer temperatures and abundant rainfall.

Primarily, the aging leaves located near the soil surface are affected, developing water-soaked spots with a gray center and a dark border. After some time, they become covered with black pycnidia. Gradually, the pathogen

6* 163 takes over the entire leaf, which dies off. Disease symptoms can appear on stems and flowers; fruits are rarely affected.

The hosts of the septoria pathogen include many weed species of the Solanaceae family.

The spread of the pathogen is favored by wet weather, as moisture is required for the release of spores from the pycnidia. The infection is also spread by water droplets. The fungus is most active at temperatures of 15–25 °C.

Control. To protect greenhouse tomatoes from septoria leaf spot, it is necessary to destroy weeds and remove all plant debris. Fungicides such as zineb, maneb, and benzimidazoles effectively suppress the disease.

Stemphylium leaf spot (Stemphylium vesicarium, S. botryosum)

This tomato leaf spot has little economic importance in European conditions. A similar disease, noted in various countries, including Great Britain, is caused by the fungus Pleospora herbarum (the perfect stage of S. botryosum). On the leaves of diseased plants, irregular small dark-brown or grayish spots up to 1 cm in diameter and of various shapes develop. Merging together, they form large areas of dry tissue, which leads to the wilting and death of aging leaves. Stems, leaf stalks, and fruits are not affected.

Control. Outbreaks of Stemphylium are linked to high air humidity in the greenhouse. Common fungicides such as benomyl and mancoceb do not suppress the disease; however, data have been obtained indicating that iprodione is effective against fungi related to the Stemphylium pathogen.

Alternaria leaf spot, early blight (Alternaria solani)

In Great Britain, this disease is very rare (although outbreaks have been noted in some cases), but in other countries, particularly North America, it poses a major danger. The fungus causes ulcers on the stem, especially severely in young plants, as well as fruit rot and leaf spotting. The latter symptom is the only one encountered under British conditions. Small, irregular brown spots first appear on aging leaves. Gradually, they grow to 1 cm in diameter, often becoming ring-like, and usually have a chlorotic border. On stems, spots with a raised gray center can develop. Spotting in the stalk area leads to fruit drop. On affected fruits, the fungus forms a multitude of black spores. High humidity is favorable for disease development, and harvest losses are directly linked to leaf dieback and fruit infection. Control. The pathogen is seed-borne; therefore, seeds should not be obtained from the fruits of affected plants. Seed treatment with thiram or iprodione before sowing yields good results. Great attention must be paid to hygiene. For effective control of Alternaria leaf spot, high-volume spraying with iprodione or vinclozolin is carried out. \ Late blight (causative agent: Phytophthora infestans) The disease is caused by a pathogen that is also the causative agent of potato late blight. | This is a dangerous disease of open-ground tomatoes in European conditions; furthermore, it can also be damaging in unheated greenhouses. The fungus affects leaves and fruits. Large spots, ranging from light to dark brown and usually with a lighter border, appear on the leaves. They expand rapidly, gradually covering the entire leaf blade. Fungal sporulation occurs on the underside of the leaves, where, on a shoot Fig. 7.15. Dark-brown spots of Alternaria solani infection with a lighter center; the disease is known as early blight. to the touch, the pathogen can be found on their surface.

Fig. 7.17. Late blight on a fruit; in the affected area: the tissue becomes firm, the fruit turns brown.

A white powdery coating, visible to the naked eye, develops on the affected areas. Stems are also frequently affected, developing dark streaking. Diseased fruits lose their marketability. Large reddish-brown areas of firm tissue are visible on green fruits; as they gradually expand, they can cover the entire fruit. Fungal spores easily

In unheated greenhouses, the first signs of the disease usually appear in August, i.e., during the period when P. infestans is particularly widespread on potatoes. Late blight often affects tomato plants in the center of plastic greenhouses, where ventilation and air movement are very weak. At a constantly high temperature (25 °C), disease development ceases.

Control. The primary source of infection for greenhouse tomatoes is potatoes. If unheated greenhouses are located not far from fields planted with potatoes, standard fungicide spraying should be carried out, especially in "late blight" years. Information about the possibility of a late blight outbreak can usually be obtained from the local forecast service. If a minimum temperature of 10 °C and a relative humidity of around 75% are maintained for two days, conditions become favorable for the development of late blight.

The most effective fungicides against P. infestans are from the dithiocarbamate group (zineb, mancozeb), however, they must be applied regularly. Very good results are obtained with a mixture of metalaxyl and mancozeb, which is used to suppress late blight on potatoes. In addition, any agricultural practice that leads to a reduction in humidity will prove useful.

Southern blight (Phytophthora nicotianae var. parasitica)

This pathogen affects fruits and, in addition, causes root rot in tomatoes and damping-off. Southern blight is a common disease, often appearing in greenhouses where soil disinfestation has not been carried out. Infection occurs when soil contaminated with fungal spores lands on plants, usually with water droplets. The introduction of new cultivation technologies for tomatoes, in particular the gradual lowering of the main stem, leads to most clusters being formed close to the soil and being affected by southern blight much more frequently. Typical symptoms on fruits are brown concentric spots with a gray center, developing on grayish-brown tissue. Affected fruits often drop off.

Control. The source of infection is soil that gets onto the fruits with water splashes, especially often during hose irrigation or overhead watering. Therefore, preventive measures must be taken, such as spreading straw over the surface of the soil. If disease symptoms have already appeared on the plant, control is too late.

All affected fruits should be removed and taken out of the greenhouse. Before planting the transplants of the next tomato crop, the soil must be disinfested. In some cases, treatment with copper-containing fungicides proves effective.

This pathogen appears in the UK only in isolated cases, although it is widespread in countries with warm climates. The fungus affects both leaves and fruits, and, Fig. 7.18. Damage caused by Phoma spp. Concentric circles usually appear on the grayish-brown rotting tissue.

the most significant harvest losses occur when fruits are affected. Scattered black spots develop on the leaves, gradually increasing in size (similar to symptoms of Alternaria blight). Sometimes elongated and indistinct concentric spots can also be found on the stems. However, the greatest danger is fruit spotting, which often develops during transport. Slightly sunken spots appear at the base of the fruit, which gradually turn brown and become covered with pycnidia. The fungus survives on decaying plant residues in the soil, which serves as the main source of infection.

Control. The main control measures are maintaining labor hygiene rules and high-volume spraying with fungicides. Zineb and iprodione are most effective against the disease. Verticillium wilt (Verticillium albo-atrum and Verticillium dahliae)

Verticillium wilt can cause huge damage, although it is currently not very widespread and usually affects only a small number of plants in a crop. Besides tomatoes, the wilt pathogen also affects other greenhouse crops — cucumber, chrysanthemum, pelargonium, but not carnation: carnation wilt, which is often considered to be Verticillium wilt, is caused by a different pathogen (see p. 291). Both V. albo-atrum and V. dahliae produce very similar symptoms, but the former pathogen is often considered more aggressive. Signs of Verticillium wilt can be easily distinguished from wilts caused by root rots or water deficiency, as they appear primarily on the aging lower leaves. In the early stages of disease development, wilting can be very severe during daylight hours and noticeably weakens at night as the plants recover turgor. It takes 3–4 weeks from the appearance of the first symptoms to the death of the plant, depending on weather conditions. The root system initially appears healthy, but as the disease progresses, secondary rots begin to develop.

In a cross-section of the affected stem, pronounced browning is visible in the vascular system. It can start at the soil level and extend up the stem for a meter or more. This makes it possible to distinguish the symptoms of Verticillium wilt from a similar discoloration of the vessels that occurs with root rot, which rarely rises more than 10–15 cm above the soil level. Infection occurs through the roots, and the fungus gradually colonizes the entire plant. When affected plants are removed, root residues remain in the soil, which serve as a source of infection for the next crop. Since the infection of any part of the root system by the pathogen leads to its mass spread throughout the plant, causing death, soil disinfection to the depth of the root system development becomes especially important (as opposed to-

Verticillium wilt of tomatoes

Verticillium wilt manifests as browning of the vascular bundle, which rises up the stem and is not limited only to its lower part (up to 20 cm). The pathogen causes significant damage mainly during multiple infections, resembling root rot in its nature. The pathogen survives in the soil in the form of resting bodies and can multiply to a limited extent in some soils even in the absence of host plants.

Conidia form on diseased and especially on dead tomatoes that have not been removed from the greenhouse. They are easily carried by air currents during ventilation and by water splashes, which is why the soil of seedling hotbeds can become contaminated. In this case, plants are affected by Verticillium wilt by the time they are planted in the soil. Transmission of infection via seeds is very rare.

• Soil disinfection depth — 25 cm
• Temperature for suppression of V. albo-atrum — above 25 °С
• Interval for benzimidazole treatment — 1 month

To control Verticillium wilt, use the following protection scheme:

1. Grow resistant cultivars or graft susceptible plants onto resistant rootstocks. Before planting the transplants in the soil, remove the root system of the scion.
2. When growing susceptible cultivars, disinfect the soil to a depth of 25 cm. For early-maturing crops, less deep tillage is allowed.
3. At the first sign of disease, water the plantings with a solution of a benzimidazole-group fungicide and repeat the treatments monthly.

Fusarium wilt of tomatoes

Outbreaks of Fusarium wilt, caused by the pathogen Fusarium oxysporum f. sp. lycopersici, have become more frequent recently. It is difficult to distinguish the disease from Verticillium wilt by its symptoms. The main difference is a more pronounced chlorosis of leaves and stems, which usually develops more strongly on one side of the tomato plant. The first sign of infection is chlorosis of the lower leaves and slight wilting, which gradually covers the entire bush.

In a longitudinal section of the stem with Fusarium wilt, strong browning of the vascular bundle is noticeable, which spreads throughout the plant. In the later stages, yellow streaking may appear on the stems, although new, outwardly healthy shoots may sometimes continue to develop at the base of the bush. The pathogen is able to survive for a long time in the soil on plant residues or in the form of chlamydospores, and can also be transmitted via surface-contaminated seeds.

In Europe, races 0 and 1 of the F. oxysporum f. sp. lycopersici pathogen are found (a total of three races have been identified: 0, 1, and 2). Some commercial cultivars and rootstocks are resistant to European races. Using such resistant hybrids is the simplest way to combat the disease.

To combat Fusarium wilt, use a complex of agrotechnical measures:

1. Use resistant cultivars and rootstocks. Before planting grafted plants, the root system of the rootstock must be removed.
2. Carry out steam treatment of the soil to a depth of 25 cm for the main tomato crop and for early-maturing crops. In high-risk zones, combine steam treatment with the planting of grafted plants.
3. Apply irrigation to the growing crop with benzimidazole solutions when symptoms of the disease appear.

Chemical agents for soil tillage against Fusarium wilt are less effective and should only be used when growing resistant cultivars or rootstocks. Watering with benzimidazoles provides only a partial inhibitory effect.

Tomato cultivars resistant to Verticillium and Fusarium wilt (Ризайит охузрогит, 1.5р., [усоргзй с):

Cultivar	Verticillium*	race 0**	race 1
АБипда	+	+	+
Апве[а	-	+	+
ВеШпта	-	+	+
Сиаюо\ Сгоз$	-	+	-
Сигае[	-	+	-
Сигафо	-	+	-
Сигез{ю	-	+	-
Раопа	-	+	-
Па\н	-	+	+
ДотшЬ{о	-	+	++
БошьЬо	+	+	+
Ригащо	-	+	+
Е]5е	-	+	+
Ез{геПа	-	+	-
Еигобг!А	+	+	-
ЕигоуЦе	-	+	+
Ранеиг	-	+	+
Сапне{	+	+	-
ао14${аг	-	+	+
Ягепа ег	-	+	-
НоПапабг!4	-	+	-
Мара он 4	-	+	+
Магсащо	-	+	+
Маг е!	+	+	-
Мопа!а1	-	+	+
Мета®ю	+	+	+
Озюпа	-	+	-
Ратеа	-	+	+
Риашюо	+	+	+
ВезИпа	-	+	-
Г1апо	-	+	+
Коуаю	-	+	+
Загпа	-	+	+
ЗЫцеу	-	+	+
ЗоБею	-	+	+
Зоага	+	+	+
Зона(те	-	+	+
Зона{о	-	+	-
Тамара	-	+	+
Тагка	-	+	+
\!согез	+	-	-
\Мроза	-	+	-
МАщегЬиа	+	+	-

* Resistance is indicated by the + sign. ** Races 0 and 1 are sometimes referred to as races 1 and 2.