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.
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| 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.
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| 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.”
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| 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.”
