Effect of adult plant resistance on primary penetration of oats by Erysiphe graminis f. sp. avenae

Abstract

Selected events in primary penetration of oats by Erysiphe graminis f. sp. avenae were studied in susceptible cultivar Mariner and adult plant resistant (APR) cultivars Maldwyn and Dal. Fully expanded second- and fourth-formed leaves represented juvenile and adult plant stages, respectively. Frequency of conidial germination and penetration peg, haustorium, or papilla formation were examined to assess their roles in fungal ingress. Conidial germination was similar on all cultivars 24 h after inoculation. Germination was greater on second than fourth leaves of both Mariner and Maldwyn and was apparently not related to mildew infection type. Frequencies of penetration pegs, haustoria, and papillae varied among cultivars and leaf positions. The penetration peg category was used only to classify sites where pegs formed but no papilla or haustorium was evident, even though pegs were also observed at sites where haustoria or papillae formed. Some conidia stopped developing after peg formation, even in the absence of papillae. Haustoria were interpreted as successful penetrations; papillae were interpreted as penetration failures since they were rarely accompanied by haustoria. None of these events was associated with any particular type of epidermal cell or proximity to stomata, except that no guard cells were penetrated. Germinated conidia on second and fourth leaves of Mariner formed many haustoria and were associated with few papillae; no difference was evident in the papillae per germinated conidium (P/GC) ratios of the two leaf positions. On second leaves of Maldwyn (APR), haustoria and papillae occurred at the same frequency as on Mariner, but fourth leaves of Maldwyn supported significantly more papillae and fewer successful penetrations. Similarly, Maldwyn had the same P/GC as Mariner in leaf two but significantly higher P/GC in leaf four. Germlings on both second and fourth leaves of Dal (APR) formed few haustoria and were associated with many papillae. Dal had higher P/GC than Mariner and Maldwyn in leaf two, but was similar to Maldwyn in leaf four. Thus, papilla formation, which may limit primary penetration, was implicated in APR of Maldwyn, but not in APR of Dal.