Seedling diseases and nematodes: dangers to young plants
Pythium. Seed rot and pre-emergence damping-off diseases caused by Pythium can be major problems in cotton regions where soil temperatures at planting are less than 60 degrees F. Symptoms include root decay and death as well as light tan to dark discoloration of rotting roots and stem tissues.
Rhizoctonia. Seedling death resulting shortly after emergence is called postemergence damping-off. Rhizoctonia is usually the cause of soreshin, which refers to situations in which only stem girdling occurs. Disease lesions appear mainly on the seedling’s stem below its cotyledons or seed leaves, but they can also occur on taproots. Lesions have well-defined margins and vary in appearance from water-soaked to light tan, reddish-brown or dark brown. Infection and lesion development normally occur below the soil line, but as the seedling stem grows, lesions can be visible at the soil surface.
Thielaviopsis. Also called black root rot, Thielaviopsis causes blackening of the taproot and rotting of lateral roots, coupled with stunting of cotton seedlings. Development of black root rot is most severe when soil temperatures at planting range from 60–64 degrees F.
Fusarium wilt/root knot nematode complex. The root knot nematode (RKN) is a small, parasitic worm that injures cotton roots, allowing entry for the Fusarium disease. Root knot nematode and Fusarium wilt are grouped as a complex because they are normally found together in sandier soils. There is, however, a Race 4 Fusarium in California that occurs in heavier soils, where it can infect the cotton root system without the help of RKN. This aggressive race of Fusarium can destroy a whole stand of susceptible plants. Pima varieties are most susceptible; however, when inoculum levels in the soil are high enough, upland varieties can also be damaged by this disease. To date, this race of Fusarium has not been reported outside the state of California.