Current Conditions
Some of our earlier cotton is now moving right along and pushing out the fifth true leaf. Many fields, maybe 20%, are being replanted. Seems like we are having to bring out the sand-fighters once a week – which is a good thing from the stand point that more rains fell – just like last night (June 17). As these rains keep coming, we are increasing the soil water content in the top (2 & 8 inch) where the seedling plants are using it before it moves down into the deeper profile.
Seedling Disease
With this increase in soil moisture content and the length of time it is taking some of the seed to emerge – we are set up to encounter seedling disease. A number of different soilborne pathogens including Pythium spp., Rhizoctonia solani and Thielaviopsis basicola are involved in the seedling disease complex.
RHIZOCTONIA a.k.a. Soreshin, Rhizoc, Damping Off
The most common cause of post-emergence damping off throughout the world Rhizoctonia invades the cotton plant at soil level. It produces a sunken lesion which girdles the hypocotyl (stem), causing the seedling to collapse. In wet conditions, the lesion can extend upwards several centimeters from the soil line. Plants surviving Rhizoctonia are weakened, and they bear the mark of the stem-girdling lesion at the base of the stem (Soreshin).
Contributing Factors: Excessive soil moisture predisposes cotton seedling to infection by reducing their rate of growth. Infection occurs over a wide range of soil moisture levels.
PYTHIUM a.k.a. Root Rot
Pythium infects the seed and radical, causing seed rot and pre-emergence damping off. The seedling hypocotyl (stem) can also be affected at the soil line, causing post-emergence damping off. At later stages of plant development, Pythium may cause stunting and chlorosis.
Contributing Factors: Pythium is most damaging to cotton seedlings at low temperatures and high soil moisture content. Degree of infestation is also impacted by soil texture and organic matter.
THIELAVIOPSIS a.k.a. Black Root
Thielaviopsis is most prevalent in Texas, Mississippi, New Mexico, and the San Joaquin Valley area of California. Infection occurs at the seedling stage with roots and the portion of the hypocotyl below soil line rotting and turning black. When older plants are infected by Thielaviopsis the result is collar rot. Sign of Thielaviopsis include swelling/blackening of the tissue at the base of the stem, and fungus growing from infection sites.
Contributing Factors: Thielaviopsis is more prevalent in clay soils than sandy soils and is usually most severe under cool, wet conditions.
I have only seen two fields which are experiencing any seedling disease and both cases have been rhizoctonia (damping off) and both were in Dawson County.
Again, high soil moisture provides conditions conducive for growth of these microorganisms and may predispose the developing seedlings to infection by slowing plant growth.
