For several seasons, pest control advisers, growers and University of California researchers have been on alert for an Australian strain of fusarium wilt in San Joaquin Valley cotton.
Concerns were raised several years ago that Australian cottonseed for dairy rations may harbor a particularly virulent strain of fusarium wilt from that country.
It has not been found, but apparently a California kissing cousin has been. The California cousin has been identified as race 4 of fusarium oxysporum f.sp.vasinfectum (4 FOV for short).
University of California Extension Cotton Specialist Bob Hutmacher said at the recent California Cotton Growers Association annual meeting that this newly found race shares one particularly troubling trait with its Australian kin, it is not associated with root knot nematode.
Until now, fusarium in the valley has been linked with significant populations of root knot nematode. This new race been found in finer textured soils not associated with large root knot nematode populations.
The good news is that the Australian strain has not been found in the valley. The bad news is that its kissing cousin has been. The worst news is that this California kin takes a particular hankering to Pima cotton.
With a very large Pima acreage for the valley this year, Hutmacher and his UC Cooperative Extension farm advisors are sounding the clarion to be on the lookout early this year for 4 FOV. A special fusarium management guideline packet is available on the UC cotton Web site.
Last year all widely grown commercial Pima varieties were more seriously damaged than tested Acala and non-Acala cotton varieties. However, the uplands did show infection last year. The damage and symptoms were less.
Hutmacher said the best time to scout for this new race is earlier than when growers and PCAs start looking for weed or verticillium wilt problems.
“Fusarium affects plants as early as the one to two-leaf stage and more typically around the five to 12-node growth stages,” Hutmacher wrote in a special cotton management guideline issued about this new problem. As a vascular disease, fusarium can restrict nutrient and water uptake by the plant.
Don't wait until peak bloom or later in boll development to scout for fusarium, said Hutmacher. It is more difficult then to identify infected field areas because damaged plants may be too small to identify under canopy cover or partially recovered plants may have recovered to mask the affected area.
Hutmacher says fusarium staining looks different from verticillium staining. FOV staining is more continuous, dark brown color compared with the irregular, flecking stain of the vascular system typical of verticillium.
Visual fusarium symptoms include general wilting featuring patchy leaf yellow and necrosis that begins on the leaf margins of the lower leaves. As the disease progresses, the vascular system appears stained dark brown when cut at a diagonal.
In seedlings and young plant, cotyledons and leaves wilt and drop, resulting in bar stems. On older or mildly affected plants, lower leaves will show symptoms, but survive with reduced vigor.
It requires laboratory testing to determine if it 4 FOV. UC farm advisors and specialist are equipped to do that testing.
Identifying it is important because there is no effective control measure. The only way to prevent its spread is containment by:
Removing infected plants by hoeing them out and burning them or killing with herbicide.
Controlling fusarium-harboring weeds during the season or between crops.
Controlling equipment from infected areas moving to clean areas. Farmers are urged to clean soil and plant debris from equipment exiting an infected area.
Managing irrigation water to prevent spread from infected to non-infected areas.