Localization of plant symptoms on leaves and organs

Powdery mildew fungi most often develop on the leaf surface; colonization of the leaf underside occurs less frequently and with lower intensity. Signs of damage by pests, especially spider mites and thrips, are also more pronounced on the leaf surface.

Pathogens of downy mildew and rust sporulate on the underside of leaves but simultaneously cause moderate symptoms, similar to chlorosis on the leaf surface, directly beneath the areas of sporulation.

Signs of damage to stomatal tissues most often develop on the underside, where stomata are very numerous. A typical example of such a symptom is edema. Corking of leaves or the formation of small watery spots is characteristic of many plant species if transpiration ceases due to environmental conditions.

Deficiency of certain nutrients, such as potassium, leads to the appearance of chlorosis or necrosis along the leaf margins. Similar are the symptoms of damage from toxic concentrations of certain systemic pesticides (in this case, chlorotic tissues are often unevenly distributed) and increased water loss through hydathodes, leading to tissue dehydration and the formation of necroses. In lettuce, this leads to tipburn (or scorch). Certain bacterial vascular pathogens that secrete toxins (e.g., Corynebacterium michiganense) cause typical marginal leaf wilting, followed by the development of marginal necrosis.

Various spots are particularly likely to develop on the leaf blades at a distance from the edges. Sometimes they are concentrated in the central part of the blade, most strongly along the midrib and to a lesser extent along the secondary veins. Under the influence of sodium chlorate, initially only the veins are affected; with severe toxicosis, the entire leaf dies, and then the whole plant. Symptoms that appear consistently only on interveinal tissues are signs of nutrient deficiency or toxicity. The lack of a correlation between the localization of symptoms and the pattern of venation most likely indicates infection by a pathogen. SYMPTOMS MANIFESTING ON ONE SIDE OF A PLANT OR ON A SINGLE ORGAN

Such localization is typical for symptoms of chemical or physical damage, which can affect all plants in a planting. This happens, for example, with the accidental application of a herbicide remaining in an uncleaned sprayer. The intensity of symptoms may decrease with distance from the initial spraying site. Damage from temperatures or chemicals often manifests on one side of a plant and depends on its position relative to broken glass in a greenhouse, a stream of sunlight, or a source of chemicals.

Partial wilting begins with damage to the stem (especially at the site of disease development) at the point of lateral shoot branching or along them. In this case, signs of wilting appear on the upper leaves. Certain vascular pathogens, for example, Fusarium oxysporum f. sp. lycopersici, penetrate the vascular system and lead to unilateral wilting of the affected plant. SYMPTOMS OF WILTING OF ALL LEAVES

Such symptoms can appear within one day of contact with a growth regulator type herbicide. The plant looks wilted, although the leaves retain turgor. This plant reaction is called epinasty.

Similar symptoms develop on tomato plants in the early stages of Fusarium wilt infection. Wilting of the entire plant also occurs during the development of vascular bacterioses, root rots, and after prolonged periods of drought.

The most typical symptom of stem damage is ulceration or decay. Such areas appear along the entire length of the stem from its base to the apex.

• Development of symptoms at the base of the stem, at the soil level, indicates infection by soil-borne pathogens.
• Symptoms on internodes or affected areas located along the entire length of the stem are indicators of colonization by a pathogen of aging tissues or its introduction to the plant during maintenance.
• Streaks and stripes appear on immature and youngest stem tissues, especially when infected by a viral pathogen.
• Yellow streaking at the base of the stem, occurring on one side, is a sign of Fusarium wilt, especially on tomato and cucumber.

Discoloration of xylem vessels occurs as a result of vascular wilt infection; it differs from vessel discoloration in root rots, as it develops much higher than the soil level and is not limited to the lower part of the stem, as shown in the table below.

The development of adventitious roots at the base of the stem is an indicator of damage at or below the soil level, in the surface layer.

The most typical symptom is root rot. Pathogenic rots usually begin with the development of small lesions that spread and can gradually cover the entire root system.

Rapid general root decay is likely linked to poor soil conditions, such as waterlogging or the presence of toxic salts. Uneven development of root rot can occur as a result of insufficiently thorough soil sterilization, both during the cultivation of transplants and in the soil.

If you grow tomato transplants in sterile compost and then transfer them into infested soil, rot will appear on the new roots as they grow. In this case, the initially clean soil ball around the roots will protect the plant for some time. The disease develops quite differently if the pathogen was initially present in the seedling soil and the greenhouse soil was well disinfected — in this case, infection occurs instantly.

How to identify the source of infection in production

To accurately diagnose the disease and localize its source, analyze the entire technological chain in detail. Errors at any stage — from seed procurement to the preparation of irrigation hoses — can negate the entire plant protection system. Pay special attention to key risk factors:

• Seed. Consider their storage conditions and even the potential toxicity of printing ink on the packets, as this directly affects emergence. Some cultivars have infected seed plants from the start.
• Transplants. Monitor the conditions of their cultivation, storage after rooting, and transportation. For example, low temperatures during storage delay subsequent plant growth and provoke the development of diseases, for instance, in carnations.
• Substrate disinfection. The sterilization method must correspond to the crop rotation scheme in the greenhouse. An incorrect choice of disinfection method opens the way for a recurring outbreak of infection.
• Equipment and water. Tap water is usually clean, but water from ditches and open reservoirs often carries pathogens and toxic impurities. Containers, trays, stakes, tying material, and irrigation pipes also serve as sources of early infection.

The dynamics of the accumulation of dangerous substances in steamed substrate are as follows:

The rate of soil clearing directly depends on the microclimate. At low temperatures in the greenhouse, toxic substances break down more slowly, so the waiting period must be extended. Under normal conditions, a six-week interval is sufficient for the safe planting of transplants.