Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 20, 2000
Publication Date: April 20, 2000
Citation: Martin, F.N. 2000. Rhizoctonia spp. recovered from strawberry roots in central coastal california. Phytopathology. Interpretive Summary: In California, strawberry (Fragaria x ananassa Duch.) is cultivated as an annual crop on more than 9,713 ha, was ranked as the number 11 agricultural crop in 1998 with a value of $783 million, and represents 76% of the nation¿s total strawberry production. One of the primary production areas is in the central coastal region of the state encompassing Monterey and Santa Cruz counties, which accounted for 50% of the states production in 1997. With the exception of the small percentage of the total strawberry acreage devoted to organic production, virtually all production was done on land that was fumigated with methyl bromide + chloropicrin to control root diseases, nematodes, and weeds. In addition to controlling lethal diseases such as Verticillium wilt, soil fumigation also controls a number of nonlethal soilborne fungal pathogens that also contribute to significant losses when crops are grown in nonfumigated fields. One disease complex often associated with plants grown in nonfumigated soil is referred to as black root rot. While not currently a widespread problem in properly fumigated commercial production fields, with the impending phase out of the use of methyl bromide and alteration of current fumigation practices this disease complex may once again become a problem. One group of pathogens associated with this disease complex are binucleate Rhizoctonia spp., however, very little is known about this pathogen in the central, coastal production area of California. This manuscript addresses this lack of knowledge and describes a molecular marker system that is useful for isolate identification.
Technical Abstract: Rhizoctonia species were commonly recovered from the roots of strawberry plants growing in nonfumigated soil in the central coastal region of California. With the exception of one multinucleate isolate of Rhizoctonia solani (frequency of recovery of 0.8%), all other isolates were binucleate and were in anastomosis groups (AG) A, G, or I. AGA and AGI were recovered dfrom all four collection sites, whereas AGG was recovered from only two sites. AGA was the most commonly isolated anastomosis group, followed by AGI and AGG. Evaluating anastomosis grouping by pairing isolates recovered from strawberry with known tester isolates did not always yield a positive anastomosis reaction, even though both isolates anastomosed with other members of the same AG. Subsequent investigations with multiple isolates in the same AG from the same collection location confirmed that there was a lack of anastomosis or weak anastomosis reactions for some combinations of pairings, highlighting the need for using multiple tester isolates or molecular techniques for AG determination. RFLP analysis of a PCR amplified region of the rDNA was effective for differentiating anastomosis groups. A total of 16 RFLP groups were observed with UPGMA cluster analysis using data for the size of the amplified products and fragment sizes following digestion with four restriction enzymes. While each anastomosis group had isolates in multiple RFLP groups, any one individual RFLP group contained isolates of only a single AG. There was no consistent correlation between RFLP group and the location of isolate collection. While one subclade of AGA isolates was more virulent than other AGA isolates, there was no consistent correlation between RFLP and virulence on strawberry.