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Reproduced with permission from Valley Potato Grower©
Ted Radcliffe &
Dave Ragsdale Apunte aquí para versión en Español [X] |
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Green peach aphid is far and away the most important vector of PLRV. In our region,
spread of PLRV within the field is closely correlated with numbers of green peach aphids
present on the crop. Movement of PLRV from sources outside the field by winged aphids
appears to play a relatively minor role in the spread of PLRV. This is very different than
potato virus Y (PVY) which is moved almost exclusively by winged aphids but of many
different species. Winged aphids invade potato in late June or early July. These early arrivals may be
long distance migrants. There is no evidence that green peach aphid can successfully
overwinter outdoors in our area. Insect natural enemies, i.e., generalist predators and
parasitoids, can be very effective in holding green peach aphid numbers in check. Green
peach aphid owes much of its pest status to human intervention. When insecticides are
applied to control of Colorado potato beetle or other insect pests biological control
agents are decimated. But, green peach aphid is resistant to most insecticides. Under such
circumstances, green peach aphid populations can increase very rapidly, numbers can double
every 2 to 3 days. Thus, green peach aphid tends to be a mid to late season pest and
outbreaks are most commonly associated with intensive insecticide use. Most pyrethroid,
phosphate, and organophosphate insecticides "flare" aphid numbers. Potato entomologists have tended to consider entomopathogenic fungi of limited value in
control of aphids. Reasons advanced for this presumed general ineffectiveness are:
inadequate inoculum levels, infection being too dependent upon specific environmental
conditions, and dissemination being too dependent upon the presence of uniformly
distributed and abundant hosts. However, fungal diseases do end many aphid outbreaks and
may contribute to holding subeconomic aphid populations in check. The research reported here is not recent; it was done 10 years ago by Abdelaziz
Lagnaoui, then a graduate student with Ted Radcliffe. However, these observations are of
practical importance to potato growers, especially seed growers, and more so with the
present intensive use of protective fungicidal sprays for control of metalaxyl-resistant
potato late blight. These experiments were conducted at the University of Minnesota Agricultural Experiment
Station, Rosemount, Minnesota in 1985 and 1986. The data reported is from 1986, but 1985
results were similar. In 1986, a single experiment was conducted with insecticides applied
as split-plot treatments to produce two densities of green peach aphid. Azinphosmethyl
(Guthion®) was used to induce high green peach aphid densities; methoxychlor
(Methoxychlor®) was used to induce moderate green peach aphid densities. Green peach
aphid is highly resistant to both insecticides but azinphosmethyl has greater adverse
impact on natural enemies. Six fungicides were used: captafol (Difolatan®), chlorothalonil®), copper hydroxide
(Kocide®), mancozeb (Dithane®), triphenyltin hydroxide (Du-Ter®) and metalaxyl
(Ridomil). Each was applied at the upper rate labeled for use on potato. Control
treatments without fungicide were included in each experiment.
On 29 August, infection incidence in plots sprayed with azinphosmethyl, ranged from
2.5% (metalaxyl) to 9.6% (chlorothalonil) in fungicidal treatments and was 14.4% in the
control (Table 3). On 12 September, infection incidence ranged from 11.9% (metalaxyl) to
33.8% (chlorothalonil) in fungicidal treatments and was 53.9% in the control. We identified three species of fungi associated with green peach aphid. Pandora neoaphidis
accounted for 67% of all mycoses, Entomophthora planchoniana 22%, and Conidiobulus.
obscurus 8%. An unidentified species accounted for the remaining 3%. In the
laboratory, pathogenicity to green peach aphid was greatest for E. planchoniana,
intermediate for P. neoaphidis, and least for C. obscurus. Toxicity of fungicides to entomopathogenic fungi: Effects on the germination of
conidia and growth of mycelia of the three identified fungal pathogens were measured in
laboratory bioassays. Germination of conidia was reduced significantly by all fungicides
except chlorothalonil. Captafol, mancozeb and metalaxyl severely inhibited germination of
conidia even at 0.1X rates of application. Growth of mycelia of were significantly
inhibited by captafol, triphenyltin hydroxide, mancozeb, and metalaxyl. Azinphosmethyl and
methoxychlor caused no inhibition of growth of either conidia or mycelia. Fungal pathogenicity to aphids: Pathogenicities of primary spores of the three
fungi to green peach aphid were measured in laboratory bioassays. Both in vivo and in
vitro isolates were used. In vivo isolates were produced on site from cadavers
of aphids recently killed in the field. In vitro isolates were from the USDA
collection. Our results showed that fungi maintained on culture media (in vitro) were less
infectious than those isolated from cadavers. Pathogenicity was greatest for E.
planchoniana, intermediate for P. neoaphidis, and least for C. obscurus. Toxicity of fungicides to aphids: We used dip tests to test fungicides for
direct toxicity to green peach aphids. Most toxic were chlorothalonil, and copper
hydroxide each of which caused 21% mortality at 48 hours exposure. Least toxic were
captafol, mancozeb, and metalaxyl which caused mortality of 9, 12, and 15% mortality at 48
hours exposure. Implications: Fungicides used to protect potato from foliar pathogens can be highly detrimental to entomophthoran fungi. Thus, the potential exists for upsetting biological control and triggering green peach aphid outbreaks when certain fungicides are used. We found that fungicides varied greatly in their effects on entomopathogenic fungi. This was reflected by greatly different levels of mycoses in the field and differences in effects on both conidia and mycelia in the laboratory. The result was that late season populations of aphids in field plots differed by more than 100 fold among fungicidal treatments. It is evident that careful selection of fungicides is a must if we wish to benefit from the natural control of green peach aphid afforded by entomophthoran fungi. This consideration is especially important in seed potato production because of the role of green peach aphid as a vector of potato viruses or when growing cultivars such as Russet Burbank which are susceptible to "net necrosis" (a tuber condition that can be caused by infection with potato leafroll virus). Return to Radcliffe's IPM World Textbook Home Page. The University of Minnesota is an equal opportunity educator and employer.
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