An Improved Method for Infecting Tomato Leaves or Seedlings with Oospores of Phytophthora infestans Used to Investigate F1 Progeny

An improved, soil-free laboratory method was developed to induce late blight infection in detached tomato leaves or tomato seedlings with in vivo-produced oospores of Phytophthora infestans. Oospores were produced in detached tomato leaves infected with an inoculum mixture of A1 and A2 isolates. The infected leaves were homogenized in distilled water. The homogenates were exposed to two cycles of drying and wetting to kill sporangia, zoospores, and mycelia and, thereafter, mixed with perlite and water. Tomato leaves or seed were floated on the perlite mixture in petri dishes and incubated for up to 30 days. Results from 13 experiments showed that, in dishes containing oospores, 96 of 820 leaflets (11.7%) developed late blight lesions within 6 to 30 days; whereas, in 3 experiments with seedlings, 12 of 1,400 plants (0.9%) were blighted within 17 to 26 days. No late blight developed in leaflets or seedlings maintained in dishes containing homogenates of the control leaves infected with either A1 or A2 inoculum. In all, 234 single-sporangium F1 isolates recovered from 24 leaf lesions produced by four crosses and 27 isolates recovered from 12 seedlings infected with two crosses were examined for sensitivity to metalaxyl and mating type. Results showed that, although the parent isolates were either S (sensitive) or R (resistant) to metalaxyl, most F1 progeny isolates exhibited various levels of intermediate resistance to metalaxyl. Most isolates belonged to either the A1 or the A2 mating type, depending on the cross and lesion. However, some isolates belonged to the unusual mating type A1A2 or were sterile. Current experiments are intended to elucidate the virulence and aggressiveness of these F1 progenies to tomato. This improved method may facilitate our understanding of the role of in vivo-produced oospores in the epidemiology of late blight in tomato.