Feature Article

Late Blight Fungicides Linked to Increases
in Green Peach Aphids in Potato

Potato growers are well aware of Phytophthora infestans, the causal agent of potato late blight, the same disease that caused the Irish potato famine in the late 1840's. In 1992 a new strain of the fungus (US8) was discovered in the U.S. which was resistant to the fungicide metalaxyl (Ridomil®). The US8 strain has nearly replaced the old US1 strain in the upper Midwest. For potato growers to protect their crop from late blight, fungicidal sprays are now routinely applied throughout the season with the number of applications ranging between 10 and 14 per year, a 3 to 4-fold increase to pre-1992 management practices. Previous research conducted at the University of Minnesota (Nanne and Radcliffe 1971, Lagnaoui 1990) showed a marked difference in green peach aphid numbers in relation to the type of fungicide selected to protect the crop. Some fungicides are detrimental to a group of little known parasitic fungi collectively known as entomopathogenic fungi. These parasitic fungi are keeping the green peach aphid population in check late in the season, but if these parasitic fungi are eliminated aphid populations will increase several fold over the numbers found in untreated control plots. Currently there is an ongoing research project being jointly conducted by the University of Minnesota, Department of Entomology and North Dakota State University, Department of Plant Pathology. We are studying how fungicides can be used in an integrated pest management program that conserves the entomopathogenic fungi that are necessary to prevent green peach aphid outbreaks.

To make the situation even more serious, the Red River Valley of Minnesota and North Dakota is a prime seed potato producing region with over 60,000 acres of seed potato. The Red River Valley accounts for over 35% of all interstate shipment of certified seed potatoes in the U.S. Seed potatoes must be grown under strict phytosanitary conditions to prevent the spread of various plant pathogens. With higher green peach aphid numbers, there is greater risk of late season spread of aphid transmitted viruses, potato leafroll virus (PLRV) and potato virus Y (PVY). Few growers are aware of the relationship between fungicidal sprays for late blight control and the remarkable direct impacts these potato fungicides can have on green peach aphid populations as a consequence of suppression of these entomopathogenic fungi. Higher green peach aphid numbers invariably result in greater incidence of potato viruses.

Winged aphids invade potato in late June or early July. These early immigrants 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. The green peach aphid owes much of its pest status to human intervention. For example, most growers are well aware that when insecticides are applied for control of Colorado potato beetle or other insect pests, biological control agents are decimated. But, the 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, carbamate, 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, since the increase in use of fungicides beginning in 1993, growers have observed small dead areas in otherwise healthy fields. These "hot spots" are caused by a local outbreak of green peach aphids with populations exceeding 1000 aphids per leaf! Are hot spots associated with the depletion of entomopathogenic fungi? We cannot be certain of this cause and effect but it seems reasonable to assume that if entomopathogenic fungi, in concert with other natural enemies, are eliminated that green peach aphid numbers will increase exponentially.

In 1997 we planted potatoes at the University of Minnesota Agricultural Experiment Station, Rosemount, Minnesota (nonirrigated production) and at the Central Lakes Agicultural Center, Staples, Minnesota (irrigated production). The purpose of the research was to screen various fungicides and investigate the impact of single and combination treatments (mixtures of two or more fungicides or two different fungicides used on alternate weeks) on entomopathogenic fungi. Our goal was to determine which of the commonly used potato fungicides best conserve green peach aphid entomopatho-genic fungi. Selecting fungicides which have the least impact on these natural enemies should prevent green peach aphids from increasing to damaging levels and keep PLRV spread to a minimum.

Four entomopathogenic fungi were found in 1997. The species differed between sites (Rosemount and Staples) but we don't know if this is due to location, year, or production system. At Staples, the irrigated production system, the predominate entomopathogenic fungi were Entomophthora planchonicana (60%), Conidiobolus obscurus (23%) and Pandora neoaphidis. In contrast, at Rosemount the predominate species was P. neoaphidis (48%), followed by C. obscurus (24%), Zoophthora radicans (9%) and E. planchonicana (8%). The work done by Lagnaoui (1990) was conducted at Rosemount in 1986 where he found P. neoaphidis (67%), E. planchoniana (22%) and C. obscurus (8%) comprised the entomopathogenic fungi complex.

Preliminary findings. Fungicides used to protect potato from foliar pathogens can be highly detrimental to entomo-pathogenic 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 and that these effects were location dependent. Laboratory studies are currently underway to determine if in vitro effects mimic those effects observed in the field. The results so far show that late season populations of green peach aphids in field plots differed greatly 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 entomopatho-genic 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.

- David Ragsdale, Edward Radcliffe, and Jorge Ruano-Rossil, Department of Entomology, University of Minnesota; Neil Gudmestad and Gary Secor, Department of Plant Pathology, North Dakota State University

References cited:

Lagnaoui, A. 1990. Effects of potato fungicides on entomophthoraceous fungi and the population dynamics of the green peach aphid, Myzus persicae (Sulzer). M.S. Thesis, University of Minnesota.

Nanne, H. W. and E. B. Radcliffe. 1971. Green peach aphid populations on potatoes enhanced by fungicides. Journal of Economic Entomology 64: 1569-1570.

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