Fusarium oxysporum f. sp. vasinfectum (FOV) is a soil-inhabiting fungus. FOV spores germinate when in the proximity of host plant roots. The fungus spreads from roots to stems through xylem (water-conducting vascular tissue), produces spores (with spore quantity depending in part on plant susceptibility) and can partially plug up xylem so that plants cannot transpire water.
When infected, plants die of the disease, or infected live plants are shredded and turned under at season end. FOV colonizes dead tissue and survives on plant debris and in the soil. In addition to susceptible cotton varieties, FOV can survive on root surfaces and tissue of many weeds and other plant species. Spores can survive for years in the soil. FOV becomes a permanent soil resident once introduced and, essentially, cannot economically be eradicated through known chemical or biological treatments or crop rotations.
FOV can sustain itself on roots of many crops and weeds, but FOV races specific to cotton (such as Race 4 FOV) will not cause disease in other plant species.
Scouting fields for disease
The best time to scout for FOV Race 4 is in the spring when cotton plants have 2-7 nodes. Definitely scout before first bloom.
“Blank” areas that are either devoid of plants or contain dying plants will be evident. The size of affected areas will depend on how long the inoculum has been accumulating and the susceptibility of the current variety and previously planted varieties. First-year infestations typically result in a few affected plants in a few feet of row. Second- and third-year infestations will leave expanded blank areas in adjacent rows and down the field as inoculum spreads.
Surviving plants and plants around periphery of “blank” areas will have wilted leaves, splotchy foliar chlorosis and/or necrosis, and nearly continuous, dark-brown vascular staining in the taproots.
Vascular staining is most evident in taproots. When evaluating vascular systems for staining, do not cut stems. Instead, uproot plants with a 5-to-8-inch section of taproot intact, then slice the roots lengthwise. Look for dark-brown vascular staining in roots.
FOV vs. Verticillium wilt
Verticillium symptoms appear later in season, usually mid- to peak-bloom and later. Compared to FOV, Verticillium vascular staining is generally lighter in color, more discontinuous and usually evident in lower stems (2-to-6 inches above cotyledon node), as well as in taproot tissue.
Identifying Race 4 FOV
Symptoms of all races of FOV look similar, so confirmation of Race 4 can only be made using a properly informed plant pathology laboratory that evaluates plant tissue from suspect plants. While techniques exist for evaluation of Race 4 FOV in soil, those tests are expensive, labor intensive and not currently a suitable method for evaluation of sites.
Once race 4 fov is confirmed
To the degree possible, destroy remaining infected plants in affected areas as well as immediate adjacent rows. That practice will help prevent inoculum buildup, and is particularly important if growing a susceptible cotton cultivar. Do not bury infected plants. Instead, pull up those plants and burn or compost them at high temperatures.
Mitigate impact of FOV Race 4
Carefully note locations of any confirmed or likely infested areas. A hand-held GPS unit can provide GPS coordinates of infected areas. Monitor fields seasonally for changes in size of affected areas.
Plant resistant/tolerant varieties
Growing susceptible varieties increases inoculum levels in the soil. Unfortunately, many commercial cotton varieties (Pima and Uplands/Acalas) are moderately to highly susceptible to Race 4 FOV. Do not grow varieties known as highly susceptible in confirmed Race 4 FOV fields or even near an infested site.
In fields identified with Race 4 FOV, plant only varieties with the highest levels of tolerance. Variety tolerance levels are identified in UC/USDA screening trials. Those trial results are posted on the University of California cotton website (http://cottoninfo.ucdavis.edu).
Infields considered to be absent of Race 4 FOV, continue to plant varieties with the desired quality and yield characteristics, but continue to scout for FOV disease symptoms.
Based on experience with other FOV races, rotating to nonhost crops or summer fallow will likely reduce inoculum survival (particularly multiyear rotations), but will not eradicate FOV. Upon replanting of susceptible cotton varieties, the disease will quickly return to damaging levels.
Containment options to limit spread
If affected area is large, plant resistant varieties or alternative crops.
If affected area is small, consider:
- Soil solarization. Can be alone or in combination with fumigant chemicals, especially with double film layer for one to two-plus months of duration
- Long-term fallow (which must include hot summer period and be kept weed-free).
- Summer flooding. A flood duration of several months or more is likely required.
- Soil fumigants. Use chemicals with some fungicide activity and fumigant properties for spot treatments. Check with UC/USDA-ARS for suggestions.
FOV can spread through movement of infested soil, infested plant debris and infected seed. Researchers believe that much of the field-to-field, localized introduction of FOV-4 inoculum has occurred with practices that move soil.
Limit movement of soil
- Pressure-wash implements, sprinkler pipe, machinery (e.g., harvesters) coming from farms or fields where FOV infection is unknown.
- In FOV-infested fields, avoid landplaning or other leveling activities that move soil. For weed control, consider practices that reduce soil movement, such as reduced-till or rolling cultivators instead of knives, etc.
- Restrict irrigation tailwater movement from infested fields. If tailwater is filled with suspended soil particles, those particles could contain spores of the pathogen.
- Limit equipment and personnel traffic on tires, equipment and boots through FOV-infested areas of fields, particularly when soil is wet and easily picked up and moved on tires and implements.
Infested plant material
Avoid any land applications of:
- Gin trash from known infested fields, particularly if not well-composted
- Manure from cattle corrals using gin trash for bedding
- Manure from cattle fed with cottonseed from known FOV-infested fields, particularly if not well-composted
Seed producers should scout seed production fields for FOV Race 4. This scouting of seed fields is best done when plants are young (from 2-leaf through 7-leaf stage). To prevent the possibility of infected seed entering the production system, do not allow any seed production within, or next to, known infested fields.
For more information
UC Extension: http://cottoninfo.ucdavis.edu
CA Cotton Growers Association: www.ccgga.org
Bob Hutmacher, University of California Cotton Specialist, 559-260-8957