Iam. Saeed et al., DISEASE PROGRESS BASED ON EFFECTS OF VERTICILLIUM-DAHLIAE AND PRATYLENCHUS PENETRANS ON GAS-EXCHANGE IN RUSSET BURBANK POTATO, Phytopathology, 87(4), 1997, pp. 440-445
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.