Seedling Diseases

Pythium causes water-soaked lesions on the roots or hypocotyl of infected seedlings. Lesions may collapse and turn light brown. Girdled plants often die. Pythium may rot seeds and seedlings before germination or emergence through the soil.

The main symptoms of soreshin, caused by Rhizoctonia solani, are sunken lesions on the hypocotyl, which are oval- to irregular-shaped and reddish brown. Girdled plants often die.

Black root rot, caused by Thielaviopsis basicola, is characterized by a dark brown to black discoloration and reduced diameter of the taproot of infected seedlings. As the plant ages, healthy tissue replaces decayed tissue, but affected plants may be stunted.

Virulent strains of Fusarium oxysporum f. sp. vasinfectum, the cause of Fusarium wilt, can also kill seedlings. Laboratory analysis is often needed for correct diagnosis of the presence of Fusarium spp. and the exact race of Fusarium.

All seedling diseases are caused by soilborne fungi common to most cultivated soils. The diseases are generally more severe during cool, damp weather that delays seedling growth. As cotton plants grow, they become more resistant to attack.

To reduce seedling diseases, make sure that conditions at planting favor rapid germination and seedling growth so that cotton seedlings quickly outgrow the most vulnerable stage and infection is less likely. Fungicide seed treatments can usually prevent severe losses caused by seedling diseases as long as growing conditions are reasonably good.

Always use the highest-quality seed you can afford. If possible, select seed that has shown a high rate of germination in a cold test. If you must use lower-quality seed, plant as late as possible to allow the soil to warm up. Regardless of seed quality, never plant if rain or cold weather (i.e. less than 15DD) is expected during the 4 or 5 days following planting. Follow UC Cotton Planting Forecasts issued during the planting season to plant into the most optimal conditions.

Use an adequate seeding rate so that the loss of a few plants to seedling diseases will not leave skips that must be replanted. Don't plant deeper than 2 inches because excessive depth delays emergence and exposes more hypocotyl surface to invasion by fungi.

Soil that is too wet at planting or during germination favors seedling diseases. To avoid excess moisture, allow preirrigated beds to drain adequately before planting, and don't irrigate up the crop during cool weather. Firming wheels on planters operated in wet soil often create a shallow compacted layer that aggravates seedling disease problems. Roots growing through compacted layers may develop constricted, weakened areas vulnerable to infection by fungi and may restrict growth later in the season.

Rotate cotton with sorghum and small grains to reduce inoculum of Pythium and Rhizoctonia.

Always use seed treated with fungicides effective against Rhizoctonia solani and Pythium spp. In cooler areas, especially in early plantings, it is advisable to include a material effective against Thielaviopsis basicola. Supplemental fungicides applied to the soil at planting provide extra protection when cool, wet weather is likely after planting, in fields with a history of severe seedling disease, and in fields that must be replanted due to seedling disease. Assess stand establishment to determine if your crop is growing satisfactorily.