Viral diseases of greenhouse tomatoes and measures to prevent their spread

Aspermy is rarely encountered in greenhouse tomatoes and usually affects only a few plants in a crop. The aspermy virus is also pathogenic to chrysanthemums, causing flower deformation (see p. 315). Therefore, if chrysanthemum plantings infected with aspermy are located adjacent to greenhouses, the disease also occurs on tomatoes, as the pathogen is transmitted by aphids.

The most typical symptoms of aspermy are:

• stunted growth of the main stem apex and the development of side shoots (suckers);
• the plant becomes bushy;
• yellow-green mottling appears on the leaf surface;
• mild discoloration on the underside;
• fruit size is usually reduced;
• fruits formed after infection contain no seeds.

Active control of aphid vectors prevents the occurrence of aspermy on tomatoes.

The pathogen of the disease affecting ornamental crops is rarely found on tomatoes. Its symptoms include vein clearing and pale spotting on the young leaves of affected plants. Later, the leaves acquire a bronze hue, and the apical leaves curl downwards like a boat. Pale, irregular mottling, and sometimes clear concentric circles, may appear on the fruits. The virus is transmitted by thrips, unlike the aspermy pathogen, and if dahlia or arum lily plantings are present near the greenhouse, regular thrips control measures and equally regular plant inspections are necessary to detect the first symptoms of infection.

This is a very common disease, but it is rare on tomatoes. Symptoms of infection include leaf mottling and deformation, similar to those developing from TMV infection. In all cases, regardless of environmental conditions, complete filiform leaf symptoms (shoestring) are observed on the plants. CMV is transmitted by aphids and has a wide host range, including cucumber, lettuce, and chrysanthemum. Controlling the vector usually prevents the appearance of the disease on tomatoes.

The pathogens of tomato black ring spot, tomato, arabis mosaic, strawberry latent ringspot, and raspberry ringspot are transmitted by soil-dwelling nematodes and can affect greenhouse tomatoes. Sometimes, albeit very rarely, natural infection also occurs. When infected with tomato ringspot, spots appear on the leaves, and black streaking appears on the stems and leaf petioles. Young plants sometimes die, but if they survive the infection, they grow normally, although a slight mosaic is visible on them. These diseases do not pose a serious problem in greenhouses where regular soil disinfection is practiced.

Small, often angular, silvery areas appear on the leaves, and the same discoloration appears on more mature plants.

Fig. 7.27. Damage caused by a sublethal dose of a growth regulator type herbicide: a — typical vein arrangement as a result of 2,4-D and MCPA action:; 6 — severe deformation of cotyledons caused by 2,3,6-TBA. ... of all leaflets of compound leaves. In case of very severe damage, fruit set does not occur. As a rule, the longer the growing season of the crop, the more severe the silvering damage, although in hotbeds for seedlings, disease symptoms intensify at lower temperatures. Early-planted tomatoes are, accordingly, most severely affected by silvering.

The disease is genetically controlled, with some cultivars being significantly less susceptible to damage, and some cultivars even being "resistant." Sucker removal (pruning) is sometimes not performed on diseased plants so that side shoots developed from healthy tissues can subsequently replace the damaged apex, which is then removed.

In some cultivars, unilateral leaf deformation begins, somewhat resembling the "fern-leaf" symptoms of TMV infection; however, such plants do not recover. Sometimes normal shoots appear on the unaffected side of the plant, and this compensates for the losses resulting from unilateral deformation. This type of damage is characteristic of cultivars that do not exhibit silvering symptoms, such as Sigosgos BB.

Leaf deformation can also be the result of the application of growth regulator type herbicides. For example, 2,4-D, MCPA, and 2,3,6-TBA in sublethal doses cause severe plant deformation. Typical of damage by these herbicides is a change in the venation pattern of the leaflets: the veins become more parallel and converge to a single point at the apex. Often, the youngest leaves become cup-shaped. The fruits of affected plants elongate, become plum-shaped, and a noticeable "beak" appears at the apex. Some herbicides, such as mecoprop, cause proliferation — the development of adventitious roots along the stem without leaf deformation.

Blossom-end rot

This is a common non-pathogenic disease of tomato grown in polyethylene troughs or bags. Sunken, circular brown spots appear on the blossom end of the fruit; affected fruits turn black inside and ripen prematurely. Blossom-end rot of this type is caused by a deficiency of calcium, which occurs when plants dry out during fruit development. Generally, the appearance of blossom-end rot can be prevented by ensuring a uniform water supply to the plants, although sometimes it becomes necessary to use calcium nitrate spraying. Certain cultivars, such as Zopaine, are highly susceptible to this rot.

Causes of spots and uneven ripening of tomato fruits

The appearance of spots on tomato fruits in a greenhouse signals errors in care or the development of dangerous diseases. Black and brown circular spots on the surface are a clear sign of the development of blossom-end rot. If the fruits ripen unevenly, forming large yellow and green spots on the sides or at the base, this indicates a complex of other problems. It is important for an agronomist to promptly identify the cause of the defect to save the marketability of the harvest.

Uneven coloration of fruits is usually caused by the following factors:

• infection of plants with tomato mosaic virus (ToMV);
• lack of available potassium in the soil;
• very high daytime temperatures, which cause the fruit tissue to heat up unevenly;
• severe defoliation of the bushes, depriving the trusses of natural protection from sunlight.