ABSTRACT
ABSTRACT Pathogenic variation in Colletotrichum gloeosporioides infecting species of the tropical pasture legume Stylosanthes at its center of diversity was determined from 296 isolates collected from wild host population and selected germ plasm of S. capitata, S. guianensis, S. scabra, and S. macrocephala in Brazil. A putative host differential set comprising 11 accessions was selected from a bioassay of 18 isolates on 19 host accessions using principal component analysis. A similar analysis of anthracnose severity data for a subset of 195 isolates on the 11 differentials indicated that an adequate summary of pathogenic variation could be obtained using only five of these differentials. Of the five differentials, S. seabrana 'Primar' was resistant and S. scabra 'Fitzroy' was susceptible to most isolates. A cluster analysis was used to determine eight natural race clusters using the 195 isolates. Linear discriminant functions were developed for eight race clusters using the 195 isolates as the training data set, and these were applied to classify a test data set of the remaining 101 isolates. All except 11 isolates of the test data set were classified into one of the eight race clusters. Over 10% of the 296 isolates were weakly pathogenic to all five differentials and another 40% were virulent on just one differential. The unclassified isolates represent six new races with unique virulence combinations, of which one isolate is virulent on all five differentials. The majority of isolates came from six field sites, and Shannon's index of diversity indicated considerable variation between sites. Pathogenic diversity was extensive at three sites where selected germ plasm were under evaluation, and complex race clusters and unclassified isolates representing new races were more prevalent at these sites compared with sites containing wild Stylosanthes populations.
ABSTRACT
Gueroba (Syagrus oleracea (Mart.) Becc.), a member of the family Arecaceae, is an important native palm tree in central Brazil and has great potential as a cultivated crop and ornamental plant (1). In July 1999, anthracnose symptoms were observed on several gueroba plants growing in a field in Planaltina, Federal District. Leaves showed spots or small circular to irregular, brown-to-black necrotic lesions, which generally coalesced as symptoms progressed. Tissue sections from leaves with symptoms were plated aseptically on potato dextrose agar (PDA). A fungus was consistently isolated from leaves of symptomatic plants and was identified as Colletotrichum gloeosporioides (Penzig) Penzig & Sacc. by P. F. Cannon of CABI Bioscience, Egham, UK, where a culture (IMI 384186) has been deposited. An isolate was tested for pathogenicity by inoculating 20 plants at the two-leaf stage by placing a mycelial PDA plug from a 10-day-old fungal culture on a previously wounded leaf. Noninoculated plants served as controls. Plants were enclosed in a plastic bag and incubated for 4 days at 26 ± 2°C and ≈100% relative humidity. Ten days after inoculation, plants developed leaf lesions similar to those observed in the field. Symptoms did not develop on the control plants. C. gloeosporioides was reisolated from 95% of the inoculated leaves, completing Koch's postulates. The pathogenicity test was repeated with the same results. To our knowledge, this is the first report of C. gloeosporioides causing anthracnose on gueroba in Brazil. Reference: (1) H. Lorenzi. Page 288 in: Árvores Brasileiras. Plantarum. Nova Odessa, Brazil 1998.
