Pathological and molecular genetic variation in the interaction between Sporobolus spp. and Bipolaris spp.

Members of the Sporobolus indicus complex, particularly Sporobolus indicus (L.) R. Br. var. major (Buse) Baaijens (Giant Parramatta Grass; GPG) and S. pyramidalis P. Beauv. (Giant Rat's Tail Grass; GRTG), are serious pastoral weeds in coastal areas of Australia. Ovariicolous fungal pathogens from th e genus Bipolaris Shoem. are agents of the disease false smut on these plan ts. The hosts are morphologically plastic and variation in disease suscepti bility may affect the use of the fungus as a biological control agent. Vari ation was investigated by screening 22 clones representative of 5 taxonomic entities within the S. indicus complex with 39 fungal isolates representat ive of 4 Bipolaris species. Both weed entities were highly susceptible to B . ravenelii and B. crustacea and only slightly susceptible to B. cylindrica and B. australis. The converse was true for other Sporobolus entities test ed. Genomic DNA was extracted from 20 Sporobolus clones (genotypes) and the RAPD technique used to generate an index of variation. This technique was able to separate the members of the complex into taxonomic groupings. There was only a 2% difference in banding patterns of S. indicus var. major clon es collected in New South Wales. Variation amongst clones of S. pyramidalis was greater (15%), indicating a more genetically diverse plant population. The more variable genetic background of S. pyramidalis is more likely to p rovide variation in susceptibility than that of S. indicus var. major.