Fp. Wong et Wf. Wilcox, Comparative physical modes of action of azoxystrobin, mancozeb, and metalaxyl against Plasmopara viticola (grapevine downy mildew), PLANT DIS, 85(6), 2001, pp. 649-656
The physical modes of action of azoxystrobin, mancozeb, and metalaxyl were
evaluated on grapevine seedlings using Plasmopara viticola as a model patho
gen. The protectant, postinfection, postsymptom, translaminar, and vapor ac
tivities of azoxystrobin were evaluated at a rate of 250 mug/ml. Azoxystrob
in provided 100% disease control when applied 1 to 5 days before inoculatio
n. Postinfection applications of azoxystrobin had little effect on the inci
dence of disease, but colony area and sporulation from the resultant lesion
s was reduced by 47 and 96%, respectively, relative to the check treatment
when applied up to 5 days after inoculation. Postsymptom applications (6 da
ys after inoculation) of azoxystrobin resulted in an 85% mean reduction of
resporulation from diseased tissue relative to the check when seedlings wer
e evaluated 1 to 14 days after treatment. Translaminar activity was greates
t when the upper surface of the leaf was treated 7 days before inoculation
of the lower leaf surface (94% disease control), in contrast, control was <
50% when leaves were similarly inoculated 1 and 3 days after treatment. Vap
or activity was not pronounced, providing maximum reductions of 5, 11, and
37%, with regard to incidence, colony area, and sporulation, relative to th
e check when seedlings were treated 1 to 7 days before inoculating adjacent
, untreated leaves. Comparatively, mancozeb (1,790 <mu>g/ml) provided compl
ete control of the disease when applied 1 to 5 days before inoculation, but
showed little postinfection activity in reducing disease incidence, althou
gh it exhibited moderate to high antisporulant activity when applied in pos
tinfection and postsymptom modes (mean reductions of 38 and 89%, respective
ly, compared with the check treatments). Metalaxyl (260 mug/ml) also provid
ed complete control of the disease when used in protectant mode, and also w
hen applied 1 day after inoculation. Applications at 3 to 5 days after inoc
ulation provided substantial reductions in disease severity and sporulation
(mean reductions of 46 and 94%, respectively, compared with the check trea
tments). and postsymptom applications resulted in a mean 84% reduction in r
esporulation. Collectively, the results of this study illustrate the unique
physical modes of action for azoxystrobin in comparison to that of two tra
ditional protectant and systemic fungicides, and provide information on how
azoxystrobin and other strobilurin fungicides with similar physical modes
of action should be best used in disease management programs.