THE LEAF INFECTION PROCESS AND RESISTANCE TO PLEIOCHAETA-SETOSA IN 3 LUPIN SPECIES

In held experiments in Western Australia, Lupinus albus cv. Kiev Mutan t was very susceptible, L. angustifolius cv. Yorrel was susceptible, a nd L. luteus cv. Motiv 369 was resistant to brown spot caused by Pleio chaeta setosa. The process of leaf infection by P. setosa was examined on these 3 Lupinus species and cultivars. Field infection occurred on young plants almost exclusively on the lower side of leaves from coni dia splashed from the soil. More conidia per cm(2) leaf surface were d eposited on Kiev Mutant than on Yorrel or Motiv 369. In humid chambers at 15 degrees C, 94-98% of conidia germinated on all species after 24 h. On all species, appressorial formation began 6 h after inoculation , but fewer appressoria were formed on Motiv 369 (27% of germinated co nidia) than on the other 2 species (average 55%) after 24 h. Leaf pene tration was directly through the cuticle via appressoria with few stom atal penetrations. Efficiency of lesion formation (expressed as the nu mber of lesions produced per 100 conidia deposited) was greater on Kie v Mutant and Yorrel (8.3 and 7.2%) than on Motiv 369 (2.3%). More lesi ons per cm(2) leaf area were produced on Kiev Mutant (24) than on Yorr el (0.7) or Motiv 369 (0.3), but average lesion size was similar in ea ch variety. Lesions expanded from 200-250 mu m at 3 days to > 500 mu m radius at 16 days after inoculation. During lesion expansion, P. seto sa hyphae grew 50-200 mu m beyond the border of necrotic lesions on Ki ev Mutant, remained at the border of lesions on Yorrel, and were 100-2 00 mu m inside the border of lesions on Motiv 369. Lesions on the susc eptible cultivars Kiev Mutant and Yorrel (but not Motiv 369) were surr ounded by a large chlorotic halo up to 2 mm from the outside necrotic lesion border. More lesions per cm(2) leaf area were required to cause defoliation in Motiv 369 than in Kiev Mutant and Yorrel. Brown spot r esistance in Motiv 369 is expressed as a reduction in defoliation. Thi s appears to result from reduced attachment of conidia, reduced effici ency of infection and lesion formation, restricted growth of mycelia i n lesions, reduced chlorosis, and delayed leaf senescence. Fewer conid ia were produced on defoliated leaves of Motiv 369 in field plots.