Fusarium Wilt in Dry Beans -1999

Progress Report to the Nebraska Dry Bean Commission March 19, 1999

Project Leader: Gary Yuen
Department of Plant Pathology
University of Nebraska - Lincoln

Cooperators: Dermot Coyne, Dept. of Horticulture, UNL
Eric Kerr, Panhandle Research and Extension Center (Emeritus)
James Steadman, Dept. of Plant Pathology, UNL
Howard Schwartz, Dept. of Bioagricultural Sciences and Pest Management, Colorado State Univ.

Project Background and Objectives:
Fusarium wilt, also commonly called Fusarium yellows, has been a sporadic disease problem in western Nebraska dry bean, but it has become more prevalent in Colorado fields. The fungus, Fusarium oxysporum f. sp. phaseoli (FOP), is the reported causal agent. Plant pathogens in the species F. oxysporum typically cause wilt symptoms. The disease in bean, however, is diagnosed primarily by a yellowing of the foliage beginning with the lower leaves. On occasion, wilt and drying do follow. Yellowing of bean foliage can result from infection by other pathogens such as Fusarium solani. One symptom that distinguishes Fusarium yellows is a light-brown discoloration of the vascular tissues that is evident when the crown and stem is split longitudinally. In recent years, Fusarium yellows has been notable in bean plantings at the Mitchell experiment station. Symptoms on susceptible lines were unusually severe, with extensive wilt and drying of foliage occurring within days after the onset of chlorosis. Several races of the pathogen, differentiated on the basis of symptom severity on select cultivars, exist throughout the world, but only one race has been reported in Colorado. In Dr. Dermot Coyne's evaluations of dry bean lines at the Mitchell Station, the reaction of some cultivars differed greatly from reactions reported in Colorado. For example, cultivars 'Chase', 'Beryl', 'Bill Z' and Pinto UI114 were resistant at the Mitchell Station but are considered to be susceptible to highly susceptible in Colorado. The atypical results raises the possibility that the disease observed at the Mitchell Station may have been caused by a new race of FOP or caused by pathogen(s) other than FOP. Because of sporadic observation of the disease in Nebraska, difficulty in visual diagnosis, and lack of confirmation by isolation, the causal agent and the distribution of the disease in Nebraska bean production areas is not known.

This is a summary of results from the first year of a two-year project having these objectives:
1. To confirm the involvement of Fusarium oxysporum f. sp. phaseoli (FOP) in severe wilting of dry bean in Nebraska.
2. To determine the prevalence of Fusarium wilt in western Nebraska bean fields.
3. To compare isolates of FOP from Nebraska with those from Colorado for race identity and pathogenicity to dry bean cultivars.

Progress in determining the causal agent:
A greenhouse experiment was conducted to compare bean cultivars for resistance and susceptibility using soil from a field with severe wilt symptoms (Bench E5, Mitchell Station). In previous trials in that field, Pinto 'Chase' and UI114 were resistant while P93-220 was susceptible. Great Northern 'Beryl' and 'Prarie' were resistant and susceptible, respectively. The cultivars exhibited the same responses when grwon in the greenhouse as in the field. Interestingly, Pinto UI114 was reported in the literature as the universal suscept to all known FOP races.

A field comparison of two bean cultivars was conducted at Bench E6, Mitchell Station. The relative levels of resistance-susceptibility noted in previously years was confirmed.

The greenhouse and field plantings provided the materials for observation of internal and root symptoms and for isolation of potential causal agents. Some, but not all, plants with chlorosis and wilt symptoms exhibited vascular discoloration and yielded Fusarium oxysporum in the isolation. Root rot symptoms also were evident in some of the diseased plants. F. solani, Rhizoctonia, and Pythium were isolated consistently from roots. All of the F. oxysporum isolates were tested for pathogenicity on the universal suscept, UI114, and on P93-220, which was very susceptible in the field, but none produced symptoms in repeated experiments. In constrast, several Colorado isolates of FOP produced severe wilt symptoms in the same experiments.
Progress in determining the prevalence of Fusarium wilt in western Nebraska bean fields:

Preplant sampling of soils from bean fields with possible histories of Fusarium yellows was conducted. Fourteen fields in Scottsbluff, Lyman, and Bridgeport areas were sampled. Beans were grown in the soils in a greenhouse to determine if symptoms of Fusarium yellows could be induced. Symptoms developed only in plants grown in Mitchell station soils.

A sampling of fields with wilt and other symptoms of root disease was conducted in August, 1998. Sampled were collected from 29 fields located in Scottsbluff, Morrill, Box Butte and Keith counties. Yellows and wilt were found in fields at the Panhandle Research and Extension Center, Mitchell station, near Melbeta, and near Brule. Fusarium oxysporum was isolated from some plants with wilt symptoms. Fusarium solani, Rhizoctonia, and Pythium were isolated from nearly all sites. None of the F. oxysporum isolates proved to be pathogenic when inoculated onto bean in greeenhouse tests. These results were confirmed by Dr. Howard Schwartz who conducted a parallel set of isolations from plants out of these fields and inoculated bean plants in his Colorado facilities.

Conclusions:

· Fusarium yellows in Nebraska appears to be different than that in Colorado.

· The disease is not wide-spread in Nebraska, but can it can be severe.

· The resistance of Chase to the severe wilt disease was confirmed in both greenhouse and field experiments, which differs from the response of the cultivar grown in Colorado. Therefore, there is the possibility that the causal agent active in the Mitchell Station fields is likely not the same race or species as that in Colorado.

· Thus far, we cannot attribute Fusarium yellows in Nebraska to infection by FOP. Difficulty in distinguishing this disease solely on the basis of field symptoms may be an explanation. The involvement of other pathogens, alone or in combination, also may be possible. For example, a new type of F. solani has been found to cause Sudden Death Syndrome, a serious disease on soybeans in the mid-west, that has symptoms and other characteristics similar to that of Fusarium yellows of bean. Bean and soybean isolates of F. solani are known to cross-infect. Pathogenicity tests for F. solani isolates from our experiments are underway to determine if any can cause the wilt symptoms.