Fusarium Wilt

Fusarium can cause severe symptoms in susceptible cotton varieties. These include a general wilt, which is especially evident on warm days, and yellowing and necrosis of lower leaf margins. The vascular system of infected plants is discolored brown in affected portions of the tissue. This is most apparent in the lower stem and upper taproot. The discoloration starts in the taproot, spreads into the stem, and is generally continuous in contrast to the speckling nature of the discoloration in plants affected by Verticillium wilt.

In seedlings and young plants, cotyledons and leaves wilt, may turn necrotic, and even fall off the plant, resulting in bare stems. Seedlings of susceptible Pima varieties often die and resemble plant losses caused by damping-off fungi. In mildly affected plants, lower leaves develop symptoms but plants survive, but with reduced vigor and noticeable stunting. Certain strains of the causal fungus only cause symptoms when plants are also infected with the root-knot nematode. In those cases, galls are usually prevalent on lateral roots.

Four genotypes (called races) of Fusarium oxysporum f. sp. vasinfectum presently occur in California. Races 1, 3, and 8 are mildly virulent and cause few, if any, symptoms on cotton cultivars currently in use unless the plants are also infected with root-knot nematodes. Race 1 is widely distributed in the San Joaquin Valley; races 3 and 8 are found in a limited number of fields in Tulare and Fresno counties. Race 4 is significantly more virulent than the other Fusarium races. It can infect plants and cause significant mortality even without root-knot nematodes present. Stand establishment in some fields is markedly reduced. The distribution of race 4 is not fully known, but it is becoming more common in Fresno, Merced, Tulare, Kings, and Kern counties.

The fungus sustains itself on the outer surface of roots of many crops and weeds and survives indefinitely in soil. In addition, the pathogen is seed-borne in cotton, which accounts for long-distance spread, and is also spread whenever infested soil is transported on boots, farm equipment, in flood irrigation, etc.

Control of the root-knot nematode is important to managing Fusarium wilt caused by most genotypes (races 1, 3, and 8) of the causal fungus. Nematicides, root-knot-nematode-resistant varieties (e.g., NemX, NemX-HY), or both are often necessary in fields infested with the nematode. Most Acala and non-Acala Upland varieties of cotton are moderately susceptible to race 4 Fusarium. Among currently available Pima varieties, Phytogen 800, Phytogen 802RF, and DP-360 have been identified as possessing relatively high levels of resistance to Fusarium race 4. Field evaluations are being conducted to identify additional varieties with acceptable levels of resistance.

Rotation to any crop other than cotton prevents an increase in the soil population of Fusarium but may not significantly reduce the number of spores in the soil. The fungus will sustain itself on the roots of most plants, including weeds (without causing any symptoms), and cannot be eliminated by crop rotation alone.

Always use Fusarium-free seed produced in disease-free fields at all times. Avoid moving gin trash that originated in infested cotton fields to noninfested fields. Any field operation that moves soil from one location to another can spread spores of the fungus and introduce it to other fields. Washing soil from equipment with pressurized water will help limit the spread of Fusarium and should be considered in sites where race 4 has been confirmed.

Other containment options for Fusarium race 4 include restricting traffic in affected patches, especially when the soil is wet, destroying affected plants and surrounding nonsymptomatic plants, and stopping irrigation of affected patches in order to prevent movement of infested soil. Soil solarization under clear plastic for a minimum of 5 to 6 weeks may reduce fungal populations, but will not eradicate all spores of the pathogen.

Cotton seed intended for planting should never be produced in infested fields. This is especially true in fields infested with race 4, which can cause devastating yield losses in certain susceptible Acala, non-Acala Upland, and Pima varieties.