Determination of Root Disease Problems on Dry Beans in Nebraska’s Production Areas

Report to the Nebraska Dry Bean Commission for the year July 1, 1996 - June 30, 1997

Investigators:
Eric D. Kerr, PI
James R. Steadman, Co-PI
Gary Yuen, Co-PI

OBJECTIVE:
(1)Determine the distribution and relative severity of major root diseases in the dry bean production areas of Nebraska for a basis of control recommendations and future research planning.

BENEFITS:
Root diseases of dry bean have been prevalent in Nebraska since production began here many decades ago.  Surveys in the 1970s in Scotts Bluff and Gering Valleys and Box Butte County indicated significant yield loss associated with moderate root rot severity.  Nematode injury was not included in that survey.

The 1996 root disease survey, as the first of a 2-yr survey, was conducted in a broader production area including all the Nebraska Panhandle, north central Nebraska, and southwest Nebraska-northeast Colorado.  The survey was conducted in June on young plants and again in late July & early August on well developed root systems.  There were 93 identical locations in each survey.  Thus, the occurrence of disease in early and late season indicates the need for specific fungicide seed treatments for early season root disease and for cultural practices that minimize season-long development of root rot.

Table 1.  Early and late season root rot ratings by region.

                                Root rot ratings

                                  June*          August**

NC Nebr                   16.6            1.8

SW Nebr-NE Colo    18.8            2.3

N Platte Valley             8.8            1.9

N Panhandle               24.3            1.9          

                    Mean      17.1            2.0

^* June root rot ratings were percent of hypocotyl below ground level with visible lesions.

^**August root rot ratings were 0 = healthy, 5 = dead.

Fungi identified in the visible lesions on hypocotyls were predominately species of Fusarium, Rhizoctonia and Pythium.  In the June samples, Fusarium root rot, Rhizoctonia root rot, and Pythium root rot were identified in 82, 30, and 4 percent, respectively, of the 93 survey fields.

In the August collections from the same 93 survey fields, Fusarium, Rhizoctonia, and Pythium root rots were identified in 98, 49, and 5 percent, respectively, of the samples.

In 1996, the survey clearly showed that both Fusarium and Rhizoctonia root rots were prevalent from early season.  This implies that fungicide seed treatments specific for both those diseases would be beneficial during early plant development.  The high percentage of locations with these diseases indicate that their control is of great importance to growers.  The mean root rot rating of 2.0 in August indicates that up to 12  bu/acre of seed yield may be compromised in fields with high yield potential.

Root lesion nematodes were identified in 81 percent of the 93 fields sampled in both June and August.  The mean late season population was 333/gram dry weight of root. Forty nine percent of fields had populations greater than 1000/gdw of root which is usually indicative of population densities that reduce yield in dry bean.