A survey of Arizona cotton growers will be conducted March through June 2012 to determine if the Fusarium wilt pathogen – especially the latest version called Race 4 – is detected in fields.

Race 4, Fusarium oxysporum f. sp. vasinfectum (FOV), has become problematic in California’s San Joaquin Valley. Fusarium wilt disease has never been problematic in Arizona.

Race 4 is an early-season disease on cotton.

Arizona growers can submit samples of suspect cotton.

Preliminary diagnostics of the cause of disease will be conducted in the field, University of Arizona (UA) county Extension offices, and/or in the UA Extension Plant Pathology Lab to distinguish possible FOV from other seedling diseases.

The survey is supported by the Arizona Crop Improvement Association, UA Extension, Arizona Cotton Growers Association, and the Arizona Cotton Research and Protection Council.

Fusarium wilt of cotton is caused by the FOV soil-borne fungus. Several races of the pathogen occur worldwide and only infect cotton. Some races depend on co-infection with root-knot nematode to be problematic, but Race 4 does not.

FOV is introduced into fields in soil, plant debris, and contaminated seed. It moves within fields in soil and water. The fungus infects through roots and proliferates inside the vascular tissue causing symptom development in susceptible cotton varieties.

FOV increases in the soil when infected plants are plowed down. Although detection in seed is difficult, FOV 4 can be seed borne.

Disease severity depends on the amount of FOV in the soil and cotton variety. Varietal susceptibility varies widely in Pima cotton, but Upland varieties are susceptible also.

Prevention steps include planting tolerant varieties of cotton, using seed produced where FOV 4 does not occur, cleaning equipment of any kind (including shoes) that may have been used in an infested field, and carrying soil from one field to another.

FOV also survives on many other plants without causing disease, including rotation crops and weeds, so the movement of equipment from California should be carefully monitored.

Symptoms can be found in seedlings and young plants. In seedlings, FOV 4 is manifested in seedlings at the 1-2 leaf stage as wilt and/or defoliation and collapse. Severely affected plants die.

The symptoms are similar to those of other seedling diseases in Arizona, and the cause should be determined in the laboratory.

In young plants with 4-10 nodes, symptoms may include pale yellow to tan patches on the lower leaves. When the root and lower stem are cut diagonally along portions of the root and lower stem, dark brown discoloration is observed in the vascular tissue.

Plants may be partially or totally defoliated.

Recovery depends on variety susceptibility and the fungus amount in the soil.

At this stage of growth, cotton in Arizona usually has no other disease problems except root-knot nematode so new symptoms are especially worrisome. The disease may appear in small areas that will not be observed later in the season as infected plants recover (although usually stunted) and unaffected plants fill in the gaps.

It is important to sample suspect plants at the seedling or young growth stages.

Sampling of cotton for seedling/young plant disease diagnosis should include:

digging out entire plants with a shovel to get as much undisturbed root as possible;

rinsing soil from roots and placing plants in a plastic bag; for larger plants, tops can be cut from roots and bagged separately; the more of the plant submitted the better; and

choosing plants that show symptoms, preferably in different stages but are not dead.

If FOV symptoms are observed, contact one of the UA faculty listed below for information on mailing samples or to schedule a visit for sample collection:

Mary Olsen, Extension plant pathologist, school of plant sciences, [email protected], (520) 626-2681;

Ayman Mostafa, area agent, Maricopa, Pinal and Pima counties, [email protected], (602) 827-8200 ext. 313;

Randall Norton, director, Safford Agricultural Station, [email protected], (928) 428-2432;

Linda Masters, director, La Paz County Cooperative Extension, [email protected], (928) 669-9843;

Kurt Nolte, director, Yuma County Cooperative Extension, [email protected], (928) 726-3904;

Shawna Loper, area agent, Maricopa, Pinal and Pima counties, [email protected], (520) 520-836-5221 ext. 215.