Terry Pitts, Oklahoma State University Extension IPM specialist at Altus, cautions cotton farmers to watch for disease and cold weather injury as plants emerge from the soil this spring.
Pitts says irregular emergence patterns or wilting plants later on could be Pythium symptoms.
"Pythium damage is greatest when post plant soil temperatures are low and soil moisture is high," Pitts said. "Pythium species have been isolated from more than 25 percent of necrotic hypocotyls when post planting soil temperatures averaged 61 degrees Fahrenheit or less," he says.
Pythium Ultimum is the most prevalent species found on cotton seedlings, he said.
Pythium infections appear on hypocotyls as pinpoint discolored spots to large necrotic areas that are often slightly sunken. On severely affected seedlings, the hypocotyl may be girdled by a soft, water soaked lesion or by a firmer, light brown necrotic area extended down to the collet of the plant.
Affected seedlings often die, either toppling over or remaining upright during senescence. Damage to seedling roots is usually seen by soft, light brown lesions on the root tips. On older plants, Pythium causes soft, light brown lesions on both tap and secondary roots.
Rhizoctinia also may damage cotton. Rhizoctinia is a fungus that causes cotton seed rot, lesions on the hypocotyl and root rot in cotton seedlings, Pitts says. Rhizoctonia solani can cause disease under a range of environmental conditions.
Hypocotyl lesions are generally reddish-brown in color and sunken. Root lesions can be wet areas where the root appears as a string (The surrounding tissue has rotted away.) or darker colored lesions. When more than one seedling disease pathogen is present, such as R. solani and Thielaviopsis basicola, then root lesions are dark brown in color. As the plant matures, the hypocotyl area hardens and is not susceptible to damage. Management of R. solani involves planting high quality seed, using a fungicide seed treatment with activity against R.solani, planting when soil conditions are warm and rain is not imminent and, in many cases, using in furrow fungicide at planting.
Managing these diseases requires planting at the proper time, Pitts says.
In addition, using a seed treatment that is effective against each disease is possible by using the current seed treatments present on commercial seed. Proper time for planting at Altus, Okla., is after May 15 based on historical climate information.
Producers should be aware of black root rot. The root system turns black, particularly in young plants, but the root system remains firm. It does not rot. However, if a disease complex involves several different fungi, the black color (necrosis) can be combined with actual root rot (softening of the tissue) and root death. This disease can be severe when soil temperatures are cool.
Generally, Pitts says, soil moisture is not necessary to have serious black root rot, though disease symptoms may be more severe when cool temperatures are combined with wet soil. Once the soil temperatures warm, the black root systems will be sloughed off and the roots will appear white and healthy.
An unusual exception may appear when soils are heavily infested with this fungus, or when root-knot nematode is present with T.basicola. Then, root symptoms may persist all season.
Plants with black root rot can recover and produce high yields. However, if cool spring weather is prolonged, plants may be sufficiently delayed in maturity and growth to the point that they will remain stunted all season. Management of this disease involves planting when soil temperatures are warmer. Generally, crop rotation of one to two years is not sufficient to reduce disease pressure significantly.
Proper planting conditions will help eliminate disease and environmental issues that show negative effects on cotton after planting, Pitts says. At a minimum, soil temperatures in the seed and root zone should exceed 60 degrees and the five day forecast maximum temperatures should exceed 80 degrees. Additionally, nighttime minimum temperatures should be forecast to be above 50 degrees for the following five days.
More in depth information can be found at the ntokcotton.org website.
Pitts may be contacted at firstname.lastname@example.org or 580-482-2120.
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