Fusarium Crown Rot on Yield and Persistence of Alfalfa

L. H. Rhodes

Department of Plant Pathology, The Ohio State University,

Columbus, OH 43210

Crown rot of alfalfa (Medicago sativa L.) is a disease of mature plants in which crown tissue is decomposed by microorganisms. Plants with crown rot produce fewer and smaller stems, gradually become less vigorous, and eventually die. Fusarium spp. are among the most common fungi isolated from decayed crown tissue and are generally considered to be among the principal organisms responsible for crown rot. The objectives of this research were: 1)to determine the effect of various Fusarium spp. and isolates on growth and survival of alfalfa, and 2)to develop a method for screening alfalfa populations for resistance to Fusarium crown rot.

Five Fusarium isolates from Ohio, three from New York, and one from Wisconsin were obtained from decayed alfalfa crowns or roots. Three alfalfa populations were chosen on the basis of observed differences in persistence in the field. Alfalfa populations 1 and 2 were designated as ‘persistent’ and population 3 was designated ‘non-persistent.’ Seed of each population were planted in a steam-sterilized peat-vermiculite-soil mix in 15-cm pots. Two weeks after emergence plants were thinned to 12 per pot. After 10 weeks of growth, all plants were cut back to 2.5 cm stubble height. Stubble was inoculated with a conidial suspension of an individual Fusarium isolate or with a control suspension lacking conidia. After 5 weeks regrowth plants were again cut back and inoculated similarly. No further inoculations were made. Four additional harvests were made, each after 5 weeks of regrowth. Forage dry matter yield was determined for each harvest. At the conclusion of the last harvest the number of surviving plants, stems per plant, and fresh weights of roots plus crowns was determined. Surviving plants were evaluated for crown rot (1 to 5 scale), crown symmetry, and presence of secondary crowns.

There were no significant population by isolate interactions for crown rot rating or for any measured variable. Significant differences in yield between populations were found in harvests 2 through 6. Across all Fusarium isolates, populations 2 and 1 (tentatively classed as persistent) yielded approximately 10 % and 5 % higher, respectively, than population 3 (tentatively classed as non-persistent). There were small but significant differences in crown rot between populations, with population 3 having the highest crown rot rating (4.20) followed by population 1 (4.12) and population 2 (4.04). Across all alfalfa populations the most damaging Fusarium isolates caused 15-20% reduction in total dry matter yield and 18- 23% reduction in plant survival in comparison to the uninoculated control. Data suggest that Fusarium spp. contribute to reduced yield and persistence and that resistance to Fusarium crown rot may be beneficial for long-term alfalfa productivity. Further, results indicate that stubble inoculation with Fusarium conidia can be used to induce crown rot symptoms typical of that seen in the field, and that differences in forage yield and plant survival normally observed over several years in the field can be partially simulated in the greenhouse in approximately 35 weeks.