Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2000
Publication Date: N/A
Interpretive Summary: The most cost effective means of controlling diseases in plants is the use of resistant varieties. However, many commercial varieties do not have resistance to important pathogens. In many cases it has been observed that isolates of fungi that do not cause severe disease often are infected with special RNA molecules. The transfer of these special RNA molecules from one isolate to another often results in a diminished ability of the fungus to cause disease. Diseases caused by the fungus Macrophomina Phaseolina can reduce yields by over 60% in crops including bean, sorghum, cotton and over 400 other crop species. The work reported here was an extensive screening of isolates of this fungus for the presence of these special RNA molecules. A total of 110 different isolates of the fungus, were evaluated and 21 isolates were identified that contained the special RNA molecules. Several isolates that had the RNA molecules were compared to isolates that did not have the RNAs for the ability to cause disease on sorghum grown in two different field locations. In both tests, the isolates that had the RNAs caused less disease than the isolates that did not have the RNAs. These results are of considerable importance to plant breeders, pathologists and growers. It may be possible to transfer these RNAs from weak isolates to isolates that cause severe disease. This might convert a severe isolate to a weak isolate and result in a reduction in crop losses.
Technical Abstract: One hundred and ten isolates of Macrophomina phaseolina, the casual agent of charcoal rot of many different plant species, from Mexico, Somalia and several locations in the United States were analyzed for the presence of double- stranded RNA (dsRNA). Twenty-one isolates had dsRNA elements from the United States and Mexico. These isolates could be partitioned into 15 different groups based on the electrophoretic profiles of the dsRNAs. The sizes of the dsRNAs ranged from approximately 0.4 kbp to 10 kbp and the number of dsRNAs present among the isolates ranged from one to ten. Attempts to detect virus like particles by sucrose gradient centrifugation were unsuccessful. Six isolates having dsRNA and three dsRNA-free isolates were evaluated for virulence on sorghum at two different field locations. For both locations, the isolates that had dsRNA were significantly less virulent (P<0.05) than the dsRNA-free isolates. Differences between locations were not significant (P>0.25), and the ranking of isolates based on mean lesion length indicated that the three most virulent isolates were those that were free of dsRNA. These results suggest an association between the presence of dsRNA and hypovirulence in the fungus.