Potato and tomato blight

16.07.2026

POTATO AND TOMATO BLIGHT (Phytophthora infestans)

(Late Blight of Potato and Tomato)

Potato and tomato blight appears on leaves, stems, tomato fruits, and potato kernels in the field, thus affecting the decline in yield and quality. The damage caused by this disease to plants depends greatly on climatic conditions, temperature, and humidity that prevail during the growing season.

Symptoms

Symptoms of the disease appear on the leaves, stem, flowers, fruits, and sometimes even on the tuber. The first symptoms appear on the lower leaves of the plant, in the form of yellow spots, later turning brown, which enlarge in humid weather without any specific limitation, covering a large part of the leaf, which becomes necrotic. Through the leaf stalk, the mycelium penetrates the stem and sometimes affects the drying of the plant. A yellow zone forms around the necrotic spot, while on the leaf underside, during humidity, a layer of mold with conidiophores and conidia appears. During dry weather, infected leaves curl, have dark-colored spots that do not expand, and lack reproductive organs. Symptoms of the disease also appear on the stems of sensitive plants, in the form of spots of varying sizes, with a yellow color when closed. The infected tissue lacks sufficient elasticity, breaks easily, and severely infected plants dry out completely.

On potato tubers, the signs of the disease appear as irregularly shaped, dark-colored spots with a softened inner tissue. The tissue infected by the action of saprophytic microorganisms rots. During subsequent infections, localized spots appear on the tuber where overwintering in the form of mycelium is ensured. If the infected tuber is cut open, then the infected part is easily distinguished from healthy tissue, which is dark and watery in color. The infected parts of the soil take on a completely closed color, while during storage in warehouses, trenches, and storage facilities, they continue to rot.
In tomato fruits, the fungus Phytophthora infestans causes symptoms in the form of dark-colored spots of varying sizes, appearing boiled, easily falling to the ground, and rotting in a short time. Fruits with a small infection, after harvesting, cannot be stored for a long time in storage because they rot. The symptoms on the leaves, stem, and flowers of the tomato are similar to those described for potatoes.

Pathogen and disease cycle

The cause of potato and tomato blight is the fungus Phytophthora infestans, a simple unicellular mycelium without a septum, found in infected tissue between cells, while nearby cells feed on fungi. The fungal mycelium, after the incubation period ends, forms conidiophores with conides that emerge through stomata. The method of conidia formation is specific. At the branched tip of the conidiophore, the conidium is formed; the branch continues to grow, expand, and acts by releasing the conide, while further from the branch, another conide is formed.
Conidia germinate in two ways: directly forming the germinatory tube or more often transforming into a sporangium with eight zoospores each having two flagella. The mode of conidia formation primarily depends on temperature and humidity. It has been found that at low temperatures of 10-15°C, conidia transform into sporangium, while at temperatures above 15°C they germinate directly and form the germinal tube. Conidia germinate in a water droplet, forming infectious hyphae, which penetrates into the mouthparts or, after the formation of the appressorium, penetrates the cuticle. In the presence of mycelium, plant cells are damaged and dried out, while the fungus spreads to nearby cells and tissues and, after the incubation period ends, forms conidiophores with conidia. At a temperature of 18°C and 100% relative humidity, Phytophthora infestans completes its incubation period within 7-8 days, with high reproductive capacity (SHATTOCK 1988).

Infection of potato tubers can be caused during the growing season by conidia that are washed off from plant leaves by rain. Zoospores released by sporangium penetrate plant tissue through injuries or lenticels. Mycelium develops in intracellular tissue. These plant organs can be infected after harvest by conidia found on the surface of the seed originating from potato leaves. Heavily infected grains rot, while those with mild infection serve as shelters for mycelium and simultaneously represent a source of primary infection.

The development of potato and tomato blight depends on the external environmental conditions, primarily on humidity and temperature. Phytophthora infestans forms reproductive organs at high relative humidity and temperatures of 16-22°C. At low relative humidity, less than 80% have poor vitality. The fungus overwinters in potato tubers in the form of mycelium. During mild infection, mycelium penetrates the conductive tissue of the potato sprouts that have not yet emerged on the soil surface. Later, the mycelium includes the above-ground part of the plant, forming conidiophores with conidia. These organs are released through stomata, spread by wind or rain, and on potato leaves cause new infections.

Control measures
Potato and tomato blight can be successfully controlled with the combined use of agrotechnical and chemical measures. Since Phytophthora infestans overwinters in potato tubers in the form of mycelium, care must be taken to use healthy seeds for planting. Single-leaf potato plants that are sources of infection are destroyed mechanically or with appropriate herbicides. For planting, stable cultivars of potatoes and tomatoes are used.

Copper-based fungicides or dithiocarbamates are used to protect potatoes. Treatment of potatoes and tomatoes with fungicides during humid weather should be repeated once a week. In the absence of moisture, fungal infections cannot occur, so fungicide treatments are rare. The first treatment is done when the plants reach a height of 15-25 cm.
In some countries with developed agriculture, the method of monitoring and signaling the presence of the pathogenic fungus Phytophthora infestans is successfully applied.
IADK has set up a meteorological station that records the main climatic parameters, and a special program processes the data and signals the moments when the risk of spreading infections from this disease arises. Farmers are advised to follow the official website of IADK] (www.[ [NT3]]. org) where notices and messages will be placed on days when there will be signals about the risk of disease spread, and the most appropriate time to protect plants with fungicides.