The leaf infection process and resistance to Pleiochaeta setosa in three lupin species

Abstract

In field experiments in Western Australia, Lupinus albus cv. Kiev Mutant was very susceptible, L. angustifolius cv. Yorrel was susceptible, and L. luteus cv. Motiv 369 was resistant to brown spot caused by Pleiochaeta 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 conidia splashed from the soil. More conidia per cm2 leaf surface were deposited on Kiev Mutant than on Yorrel or Motiv 369. In humid chambers at 15¦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 conidia) than on the other 2 species (average 55%) after 24 h. Leaf penetration was directly through the cuticle via appressoria with few stomata1 penetrations. Efficiency of lesion formation (expressed as the number of lesions produced per 100 conidia deposited) was greater on Kiev Mutant and Yorrel (8.3 and 7.2%) than on Motiv 369 (2.3%). More lesions per cm2 leaf area were produced on Kiev Mutant (2.4) than on Yorrel (0.7) or Motiv 369 (0.3), but average lesion size was similar in each variety. Lesions expanded from 200-250 8m at 3 days to > 500 8m radius at 16 days after inoculation. During lesion expansion, P. setosa hyphae grew 50-200 8m beyond the border of necrotic lesions on Kiev Mutant, remained at the border of lesions on Yorrel, and were 100-200 8m inside the border of lesions on Motiv 369. Lesions on the susceptible cultivars Kiev Mutant and Yorrel (but not Motiv 369) were surrounded by a large chlorotic halo up to 2 mm from the outside necrotic lesion border. More lesions per cm2 leaf area were required to cause defoliation in Motiv 369 than in Kiev Mutant and Yorrel. Brown spot resistance in Motiv 369 is expressed as a reduction in defoliation. This appears to result from reduced attachment of conidia, reduced efficiency of infection and lesion formation, restricted growth of mycelia in lesions, reduced chlorosis, and delayed leaf senescence. Fewer conidia were produced on defoliated leaves of Motiv 369 in field plots.