How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Symptoms of Fusarium wilt 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 brown. This is most apparent in the lower stem and upper taproot. The discoloration 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 and drop, resulting in bare stems. In severe cases, plants die. In mildly affected plants, lower leaves develop symptoms but plants survive, but with reduced vigor. 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.

COMMENTS ON THE DISEASE

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 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 and can cause severe symptoms on certain Pima varieties and mild to moderate symptoms on most Acala varieties. Seedlings of susceptible Pima varieties often die and resemble plant losses caused by damping-off fungi. 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, 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.

MANAGEMENT

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 and/or root knot nematode-resistant varieties (e.g., NemX) are often necessary in fields infested with the nematode. The susceptibility of most Acala varieties to race 4 is unknown. Among currently available Pima varieties, Phytogen 800 possesses excellent resistance to race 4.

Rotation to any crop other than cotton prevents an increase in the soil population of Fusarium but may not 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 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 5-6 weeks may reduce fungal populations.

Cotton seed 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 Pima varieties.

PUBLICATION

UC IPM Pest Management Guidelines: Cotton
UC ANR Publication 3444
Diseases
R. M. Davis, Plant Pathology, UC Davis

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