Symptoms and Signs
Symptoms of Fusarium wilt (also called Fusarium yellows) usually appear on medium-aged or older plants and begin as a yellowing and wilting of the lower leaves. These symptoms then progress up the plant until the entire plant turns yellow and wilts. At this stage, the yellow, wilting plants can be readily observed in the field. Plants may also be stunted, particularly if they were infected at a younger age. The plant usually dies.
Infected bean plants often have a swollen root compared to uninfected roots but otherwise may appear healthy on the surface (in contrast to Fusarium root rot caused by F. solani). Discoloration of the vascular tissue (xylem and phloem) is a diagnostic symptom of Fusarium wilt, both above and below the soil line. To see this discoloration, cut into the lower stem and look for a red-brown streaking in the vascular tissues. The discoloration is usually present in plants showing foliar symptoms and is particularly evident in the lower stem and at stem and petiole nodes. Root rots caused by other pathogens (such as Pythium spp., F. solani, Thielaviopsis basicola, and Macrophomina phaseolina) may occur along with Fusarium, causing roots to have other typical root rot symptoms as well.
In garbanzos, care should be taken to differentiate Fusarium wilt from a yellows disease caused by one or more viruses that are transmitted by aphids, such as alfalfa mosaic virus. The aphid-borne viruses cause yellowing of the plant but the color is brighter than Fusarium wilt (see photo comparisons). Cut a stem longitudinally and observe any discoloration—if it is in the center of the stem, it is likely the xylem and thus Fusarium wilt (internally, the xylem tissues stain dark-brown to almost black). If the discoloration runs along the edges of the cut stem, then it is likely the phloem and caused by an aphid-transmitted virus. A photo of the difference between xylem and phloem discoloration can be found at the bottom of page 6 in the 1958 California Agriculture article, Yellowing of Garbanzo Beans, by D.C. Erwin and W.C. Snyder.
In blackeyes, the visible separation of xylem and phloem is not as distinct as in garbanzos, so any discoloration of the general vascular tissue area in blackeyes is a good diagnostic for Fusarium wilt. To identify Fusarium wilt in blackeyes, scrape off the outside of the stem several inches above the soil line. If there is any discoloration, it is very likely due to Fusarium wilt. Cutting the stem several inches above the soil line avoids confusing Fusarium wilt with diseases that affect only the roots.
Comments on the Disease
The three Fusarium wilt pathogens are host-specific to bean types: F. oxysporum f. sp. tracheiphilum only infects blackeyes, F. oxysporum f. sp. phaseoli only infects common beans, and F. oxysporum f. sp. ciceris only infects garbanzos.
Fusarium wilt has been a major problem in California blackeyes production. Fusarium wilt is less of a major problem for common beans. For garbanzo beans, Fusarium wilt has been more widespread on the coast than in the Central Valley.
All three Fusarium pathogens can survive in the soil for a long time. Continued cropping of the same bean type will increase soil presence of the host-specific Fusarium wilt pathogen associated with that bean type. The Fusarium pathogens have also been reported to be an external contaminant of seed, which has likely facilitated the long-distance dissemination of the pathogens. In blackeyes and common beans, root knot nematodes can interact with the plant and the Fusarium wilt fungal disease. A variety resistant to infection by the Fusarium wilt pathogen will become susceptible if it is infected with root knot nematodes.
Blackeyes: Currently, variety selection is the recommended management strategy for Fusarium wilt. CB-46 is resistant to Race 3, the most common race of F. oxysporum f. sp. tracheiphilum in California, but is susceptible to Race 4, which has been identified in a few locations. If CB-46 has shown symptoms of Fusarium wilt, it is possible that Race 4 is infesting the field rather than Race 3. Under these circumstances, consider planting the CB-50 variety, which is resistant to Race 4.
If root knot nematodes are present in the field, try to choose bean varieties that are resistant to root knot nematodes as well as to the particular Fusarium wilt race present. See the NEMATODES section for a discussion on nematode resistant varieties.
Blackeye variety resistance to Fusarium wilt and nematodes
|Race 3||Race 4||Meloidogyne incognita||Meloidogyne javanica|
|CB-50||Resistant||Resistant||Strong Resistance||Moderate Resistance|
Common Bean: Many of the management practices recommended for Fusarium root rot may minimize Fusarium wilt. Resistance to Fusarium wilt has been identified in a number of bean plant materials collected from various areas (accessions) and could be incorporated into California varieties if the disease becomes a major problem.
Garbanzos: In fields with a history of Fusarium wilt, plant resistant cultivars such as UC-27, which is adapted to the Central Valley.
For all bean types: when possible, take efforts to minimize the spread of the pathogen from infested to disease-free fields via transport on farm machinery, irrigation equipment, and water. Use certified disease-free seed. Crop rotation may slow the accumulation of the pathogen in the soil, but once the pathogen is established in the field, it is best to plant resistant varieties.