DISEASE PROGRESS BASED ON EFFECTS OF VERTICILLIUM-DAHLIAE AND PRATYLENCHUS PENETRANS ON GAS-EXCHANGE IN RUSSET BURBANK POTATO

The interactive effects of concomitant infection by the nematode Praty lenchus penetrans and the fungus Verticillium dahliae on symptom expre ssion in Russet Burbank potato was studied in growth chamber experimen ts. Treatments were P. penetrans at three initial densities, V. dahlia e at one inoculum density, the combination of the nematode at these th ree densities and the fungus, and a noninfested control. Gas exchange was measured nondestructively in leaf cohorts of different ages, one t o three times weekly, with a LI-COR portable photosynthesis system. Th e single-pathogen treatments had no effect on assimilation or transpir ation rates, but joint infection had a significant impact. In concomit ant infection, photosynthesis was impaired more than transpiration, so estimates of leaf health were based on carbon assimilation rates only . Reductions in assimilation rate were apparent before the onset of vi sual symptoms. Assimilation rates decreased as much as 44% in the top, and newest, leaves of concomitantly infected plants, compared to rate s in control plants. Even so, the health of newly produced leaves did not become progressively worse through time. With light use efficiency less than 0.20 mol of CO2 fixed per mol of photosynthetically active radiation used as the criterion for disease incidence, disease progres sed acropetally from the oldest to the youngest leaves. In plants infe cted with P. penetrans (0.8 nematodes per cm(3) of soil) in combinatio n with V. dahliae, all leaves in cohorts 1 and 2 were symptomatic by 4 5 days after planting, and leaves in cohorts 3 to 6 became symptomatic at weekly intervals thereafter. For the control and single-pathogen t reatments, the first time that light use efficiency fell below 0.20 in all leaves in cohort 1 was 71 days after planting. Concomitant infect ion reduced leaf life span by about 3 weeks. Both visual and physiolog ical symptom expression were invariant to differences in initial nemat ode inoculum densities ranging from 0.8 to 2.5 nematodes per cm(3) of soil in one experiment and from 1.3 to 4.1 nematodes per cm(3) of soil in a second experiment.