Tomato pythium rot

Certain species of Pythium are capable of causing rot of numerous crops. Pythium rot, described as cottony leak or watery rot, occurs during wet weather or in poorly drained areas of fields. A common sight in vegetable gardens of all sizes, tomatoes (Lycopersicon spp.) are tender perennials that grow in U.S. Department of Agriculture plant hardiness zones 10 and 11, but are often grown as annuals throughout the world. Fungal diseases, such as Pythium, can attack these low-maintenance plants resulting in poor growth and fruit production, and root rot . However, with proper care you can prevent fungal pathogens from harming your tomato plants.

Causes

Pythium is a fungal pathogen found in the soil. It can attack tomato plants when they are overwatered, planted in diseased soil and when conditions are warm and humid. When the fruit touches infected soil or when water splashes infected soil onto the tomato, infection can occur. Certain species of Pythium also cause root rot.

Symptoms

Damping-off occurs at or before germination, causing rapid decline and death of seeds or young seedlings. On older plants, disease causes severe wilting and black stem, beginning at ground level, resulting in severe stunting or death of plants.

On fruit

Pythium generally starts as small, water soaked lesions on immature or mature fruit near or in contact with the soil. The whole fruit is involved within 72 hours, the epidermis is ruptured, and the fruit collapses. Under high moisture, a white, cottony growth may be apparent on the lesion surface.

Other parts

Seeds infected with this fungal pathogen typically won’t germinate and will decompose. Young seedlings may topple over and their stems develop mushy lesions. Water-soaked lesions appear on fruits infected with Pythium spp. and infected tissues, such as stems and roots, will begin to darken and become soft before rotting. Another sign of Pythium and root rot is a white mycelial growth with a cotton-like appearance forming on infected plant tissue. Some infected tomato plants may have stunted growth while others show few above ground symptoms.

Disease cycle

Pythium species can survive indefinitely in the soil on various organic substrates or as long-lived, thick-walled spores. When free moisture is available, sporangia and spores are produced. Fruit infection can be by means of vegetative mycelium, sporangia, or spores. The pathogen is capable of direct penetration, but wounds enhance infection. Highly succulent plants due to poor growing conditions, especially excessive nitrogen, are highly susceptible.

Control

Use a biofungicide containing the beneficial fungus Trichoderma harzianum, which works to suppress root pathogens. Apply it to newly sown seeds and transplants as a soil drench; can also be used on established plants. Avoid planting in poorly drained areas.

Cultural control

The best defense against Pythium and root rot is to implement cultural control and proper care for the tomato plants. Plant only disease-free seedlings in disease-free soils and use sterilized gardening equipment. Ensure the soil the tomatoes will grow in is well drained and avoid overwatering the plants. Overhead irrigation will dampen the leaves of the plant and increase the chance of fungal attacks. Instead, irrigate lower to the ground close to the base of the plant. Another option is to use plastic mulch around the tomato plants. This will prevent water from splashing soil onto the plant and causing the fungal pathogen to spread.

Chemical control

Fungicides containing fludioxonil, copper compounds and mefenoxam can control Pythium and root rot on tomato plants. These fungicides include pre-plant soil fumigant, soil drench applied during transplant and foliar sprays applied after the fungal disease infects the vegetable plants. The application ratio and instruction vary from one type of fungicide to another. For example, one brand of fungicide recommends spraying the infected tomato plants once every two to four weeks with a solution of 1/4 quart of the fungicide mixed with 5 gallons of water. Always follow the recommended directions found on the fungicide label of the specific brand you are using. This will increase its effectiveness and prevent unintended damage to the tomato plants.