Fusarium Wilt (Yellows) of Beans in Western Nebraska-2000

Progress report to the Nebraska Dry Bean Commission, March 20, 2000

Gary Yuen, Dermot Coyne, James Steadman, and Tom Powers
University of Nebraska - Lincoln
David Nuland
Panhandle Research and Extension Center
Howard Schwartz
Colorado State University

Fusarium wilt, also known as Fusarium yellows, is caused by the fungus, Fusarium oxysporum f. sp. phaseoli (FOP). Its external symptoms are yellowing of leaves in the late season, often followed by a pronounced drying of the foliage; discoloration of the vascular system are internal symptoms. It usually can be distinguished from diseases caused by Fusarium solani and other root-infecting fungi on the basis of symptoms, but chlorosis and wilt can occur from other causes. Positive diagnosis requires isolation of the pathogen and determination of pathogencity through controlled greenhouse inoculations. Fusarium wilt has been increasing in prevalence in bean production areas in eastern Colorado and has severely reduced yields when susceptible cultivars were planted. The disease is thought to be more sporadic in Nebraska bean production areas, but because of the difficulties in diagnosis, its distribution is not accurately known. In previous evaluations of bean cultivars conducted by Dr. Dermot Coyne at the Mitchell Experiment Station, yields in susceptible cultivars were reduced by 50 to 60% in comparison to resistant cultivars. The reaction of some cultivars in this trial differed greatly from those reported in Colorado. For example, cultivars 'Chase', 'Beryl' , 'Bill Z', and UI114 were resistant in Dr. Coyne's experiment, but were rated susceptible to highly susceptible in Colorado. This discrepancy raised several questions as to the involvement of FOP and other root-infecting pathogens, and whether or not FOP active in the Mitchell Station is the same race (as determined by virulence on indicator cultivars) as that infecting bean in Colorado. Answers to these questions and accurate information as to the distribution of the disease in Nebraska will affect future selection of cultivars and the development of disease management strategies.

This report summarizes progress in the second year of a two-year project. A series of field experiments were conducted in 1998 and 1999 at the Mitchell Experiment Station to evaluate some dry bean cultivars for susceptibility or resistance to the disease and to determine the causal agents. As nematodes have been report to increase symptom severity in other wilt diseases, the involvement of nematodes was investigated. Field experiments were supplemented with greenhouse experiments using isolates of FOP and soils collected from western Nebraska bean fields. In addition, surveys of Nebraska bean fields were conducted to assess the prevalence of the disease.

PROJECT OBJECTIVES:
The specific objectives in this project were:
1. To determine the role of Fusarium oxysporum f. sp. phaseoli (FOP) and other root-infecting pathogens in severe wilting (aka. yellows) of dry bean in Nebraska.
2. To compare isolates of FOP from Nebraska with those from Colorado for pathogenicity to dry bean cultivars.
3. To determine the prevalence of Fusarium wilt in western Nebraska bean fields.


FINDINGS AND CONCLUSIONS:

The role of Fusarium oxysporum f. sp. phaseoli (FOP) and other root-infecting pathogens in severe wilting of dry bean in Nebraska.
· Some, but not all, plants collected from field sites or greenhouse experiments showing wilt symptoms yielded Fusarium oxysporum. Most of the isolates of F. oxysporum, however, were not pathogenic (i.e., cause wilt).

· Fusarium solani,and Rhizoctonia solani were isolated consistently from roots. Pathogenic (root-rotting) isolates are being identified.

From these findings we conclude that Fusarium oxysporum f. sp. phaseoli (FOP) is not the only fungus that may causing the severe wilt symptoms in E-benches, Mitchell Station. A complex of fungi, including FOP, F. solani and R. solani is more likely involved. More research is needed on the interactions among these pathogenic fungi and what environmental conditions contribute to "yellows" symptoms.

· Nematode populations in the E-bench soils were too low to be a factor in symptom development. We conclude that nematodes are not the causal agent, nor do they appear to contribute to the severe yellows symptoms. Therefore, nematode control is not an important factor in controlling "yellows" in Nebraska.

Comparison of FOP isolates of Nebraska and Colorado for virulence on dry bean cultivars.
· In a test of Pinto and Great Northern cultivars in E-benches at the Mitchell Station in 1998, it was confirmed that the susceptibility the cultivars to 'yellows' in that site was different from that of the same cultivars inoculated with Colorado isolates of FOP (see Table 1). This points to either the causal agents and/or the environmental conditions being dissimilar.

· One Nebraska isolate of FOP differed from Colorado isolates in virulence on Pinto 'Chase' and GN 'Beryl', but the pattern of virulence of the Nebraska isolate on five bean cultivars did not entirely agree with the susceptibility of the same cultivars growing at the Mitchell Station (see Table 1).

Table 1. Susceptibility of dry bean cultivars to "yellows" at Mitchell Station and to yellows caused by Colorado and Nebraska isolates of Fusarium oxysporum f.sp. phaseoli (FOP).

· Colorado greenhouse results confirms Nebraska FOP to differ in virulence from that causing yellows in Colorado (see Table 2).

· Using a genetic profiling test (RAPD), Dr. Howard Schwartz found Colorado and Nebraska isolates of FOP to be genetically distinct.

Table 2. Susceptibility of dry bean cultivars to Colorado and Nebraska isolates of Fusarium oxysporum f.sp. phaseoli (FOP). Data courtesy of Dr. Howard Schwartz, Colorado State University.

These findings indicate that FOP isolates from Nebraska and Colorado are dissimilar in regards to virulence, i.e. different races. The results also supports our conclusion that FOP is only partly responsible for "yellows". Therefore, breeding and selection for resistance to "yellows" in Nebraska will require evaluations against a population of pathogenic fungi unique to Nebraska.

The prevalence of Fusarium yellows in western Nebraska bean fields
· FOP was isolated from the 1998 varietal trial at Mitchell ("Garden" area, former white mold nursery) and from bean plantings in the E-benches at the Mitchell Station.

· Plants with wilt symptoms collected in 1998 near Melbeta, near Brule, at PHREC, and at E-benches at Mitchell station did not yield pathogenic FOP. Root rot fungi were isolated from nearly all sites.

· 1999 survey conducted in collaboration with extension educators, bean company field representatives, and PHREC staff did not reveal any fields with Fusarium yellows confirmed through the isolation of pathogenic FOP.

We conclude that Fusarium yellows caused by FOP is not wide-spread in Nebraska. Furthermore, infection by other root pathogens can result in symptoms similar to Fusarium yellows. Further research is need to determine what conditions contribute to false "yellows".

POTENTIAL BENEFITS TO NEBRASKA'S DRY BEAN INDUSTRY:
Determination of the prevalence of Fusarium wilt, the exact race identity of the Nebraska FOP population, and the contribution of other fungal species to "yellows" symptoms provides accurate information for breeding of dry bean cultivars for yellows resistance in Nebraska. Use of cultivars with high resistance in Nebraska fields infested with the yellows pathogen will contribute to maximum yields. Identification of areas in which Fusarium wilt could be a problem provides an addition criterion for planning cultural practices and selecting dry bean cultivars.

RESEARCH TO BE COMPLETED IN THE REMAINDER OF 1999-2000 FISCAL YEAR:
Additional isolates of FOP have been obtained from the Mitchell Station. The virulence of these new isolates to dry bean cultivars will be determined in comparison to other Nebraska and Colorado isolates.

SPECIAL THANKS:
This research would not be possible without assistance from Clay Carlson (Panhandle Research and Extension Center), Christie Jochum (UNL), and
Erin Wickcliff (Colorado State University). We also thank Tony Merrigan, Rex Metzger, Jim Schild, Dick Waggoner, and Ray Weed for their time and effort in identifying fields for our survey.