A Red Flag Goes Up When Cotton Fields Develop Bare Spots & Plants Start to Wilt

With a month or more under their belts, Valley cotton growers are starting to survey their young crop for early signs of trouble.

This is the time to survey the fields and record areas that they suspect suffering from Race 4 Fusarium wilt. They need to act quickly to confirm that soil-borne fungus is the culprit and then consider management options.

Bare spots will start to develop due to Race 4 Fusarium issues.

First, let’s describe technical aspects of the soil condition called Fusarium, which causes plants to wilt. There are four types, or races, of Fusarium oxysporum f. sp. Vasinfectum in California soil. Three are rather mild and don’t cause major problems with cotton plants, unless they are also infected with root knot nematodes, according to UC IPM.

Early sign of wilting leaves on a cotton plant.

The most insidious is Race 4 Fursarium.  It can be quite nasty because it moves within fields through the soil or water. Your boots or a shovel can spread it around easily.  Also, the Race 4 Fusarium survives indefinitely in the soil.

For many years, Northern San Joaquin Valley growers didn’t have to worry about it. The problem was confined mostly to the south.

But increasingly, north area growers have been discovering the presence of Fusarium wilt in their fields, according to Dan Munk, cotton specialist with the University of California Cooperative Extension for Fresno County.

What makes matters worse is the fungus increases when infected plants are plowed down after harvest. Certain Pima and Acala varieties are susceptible.

Here’s how UC IPM describes the symptoms:

There is evidence of “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.”

Plant starting to die from the soil-borne disease.

“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.”

Field scout Damien Jelen hasn’t seen evidence of bare spots in the fledgling cotton fields – so far. In the coming week, he will be visiting more fields to assess stand development and give us a better idea how the crop is faring so far this season.

Dig up the entire plant to take a sample.

For those growers who suspect aRace 4 Fusariumtrouble spot should collect a soil sample and contact their local farm advisor about testing it. Here are some tips for taking a sample:

·         Use a shovel to dig out the entire plant without disturbing the entire root

·         Rinse soil from roots and place the plants in a plastic bag

·         Select plants that show symptoms but are not dead. Try to get plants at different growth stages

If there is a problem, UC IPM suggests a number of management strategies:

·         “Rotations 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 Race 4 Fusarium 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.”

Avoid growing seed cotton in infested field.

·         “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.”

Let’s hope only kind of Race 4 growers saw over the weekend was Sunday’s iconic Indianapolis 500 auto race.