Pests and diseases affecting potato landraces and bred varieties grown in Peru under indigenous farming system

The major pests and diseases were identified and quantified on thirteen potato landraces and three bred varieties cultivated in Peru. Late blight (Phytophthora infestans) was the primary biotic constraint affecting plants before flowering with an average severity of 24%. No other pathogens caused severe foliar disease, but black scurf (Rhizoctonia solani) was relatively common on tubers of some genotypes with incidence ranging from 4.30 to 33.33%. The viruses most generally considered important in potato seed degeneration, PVY and PLRV, were extremely rare, with 1.11 and 0.12 % incidence, respectively. Other viruses considered mild, such as PVX and PVS, were more common, with incidence of 28.23 and 22.29 %, respectively. Potato flea beetle (Epitrix spp.), potato leaf beetle (Diabrotica spp.) and Andean potato weevil (Premnotrypes spp.) were common, with incidence of 28.14, 18.75 and 13.61%, respectively. Potato landraces known as Ishkupuru, Lengua de vaca, Chaulina, Chaulina Tajacaja and Negro cayash were identified as potentially resistant to P. infestans.

RESISTANCE OF TRANSGENIC POTATOES TO ATTACK BY EPITRIX CUCUMERIS (COLEOPTERA: CHRYSOMELIDAE)

Potato plants (Solanum tuberosum, cv. Russet Burbank) genetically altered to produce the CryIIIA protein from Bacillus thuringiensis var tenebrionis were tested for susceptibility to attack by the potato flea beetle, Epitrix cucumeris (Harris), on Prince Edward Island, Canada, from 1993 to 1996. Average weekly damage to the fourth terminal leaf of transgenic potato plants was 31% lower in 1993 and 20% lower in 1994 compared with nontransgenic plants. Transgenic plants had 29% fewer potato flea beetle adults when compared with nontransgenic plants in 1994, although no difference was detected between the two plant types in 1993. Recovery of potato flea beetle pupae in the soil was noticeably lower from transgenic plants expressing the CryIIIA protein in all parts of the plant (Construct No. RBBT02-10Y1) than from either the transgenic plants that expressed the CryIIIA protein primarily in the green foliage (Construct No. RBBT04-01) or the nontransgenic plants. Adult potato flea beetles, when given a choice between transgenic and nontransgenic leaf material, fed preferentially on nontransgenic material. In contrast, when adults were not given a choice, feeding damage was similar between treatments. These results suggest that the reason for reduced feeding on transgenic potato plants by the potato flea beetle is, at least in part, due to some toxicity of the plants to immature growth stages and a preference for adults to feed on nontransgenic material when it is available.

PATTERNS OF FEEDING INJURY TO POTATO BY THE POTATO FLEA BEETLE (COLEOPTERA: CHRYSOMELIDAE) IN MANITOBA

The amount and distribution of foliar feeding injury by adult potato flea beetles, Epitrix cucumeris (Harris), were examined on individually caged potato plants grown in field plots in Manitoba. Plants were either maintained as uninfested controls, or were exposed throughout the growing season to different insect densities that mimicked the natural seasonal pattern of infestation. In 1984, a trial was conducted using cv. ‘Norland’ exposed to four different densities of potato flea beetles. In 1989 and 1990, cv. ‘Russet Burbank’ was exposed to potato flea beetles, and in some treatments, plants were exposed to early summer infestations of Colorado potato beetles, Leptinotarsa decemlineata (Say). In each trial, during the late summer period of high potato flea beetle density, the amount and distribution of flea beetle feeding injury were assessed at weekly intervals. Counts of feeding punctures in single leaflets were made from leaves in the upper, middle, and lower third of each caged plant, and these data were subjected to repeated measures analysis of variance. In each of the 3 years, increasing the number of flea beetles increased the mean number of feeding punctures per leaflet in an approximately linear fashion; however, the number of punctures per beetle varied between cultivars and years. In 1984 and 1990, the number of feeding punctures per leaflet was least in the upper third of the plants, and greater in the lower, or middle and lower, third of plants. However, in 1989, the vertical distribution of feeding punctures was relatively even. Previous feeding by Colorado potato beetles increased the mean number of flea beetle feeding punctures per leaflet and changed the vertical distribution of feeding punctures. Rainfall and temperature were correlated with patterns of flea beetle injury; injury was concentrated on lower leaflets during weeks of greater rainfall, and upper leaflets were injured most during weeks with higher average temperatures. It is concluded that flea beetles exhibit preferences for feeding in specific portions of potato plants, and that these preferences change in response to previous defoliation and are influenced by meteorological conditions. Consequently, counting feeding punctures would not be a reliable method of assessing whether control measures for potato flea beetles are justified.

THE SPATIAL DISTRIBUTION OF SPRING AND SUMMER POPULATIONS OF ADULT POTATO FLEA BEETLES, EPITRIX CUCUMERIS (HARRIS) (COLEOPTERA: CHRYSOMELIDAE), ON SMALL PLOTS OF POTATOES

The spatial distribution of spring and summer populations of the potato flea beetle, Epitrix cucumeris (Harris) (Coleoptera: Chrysomelidae), on potatoes (“Superior” variety) was studied in 1987 and 1988. Slopes of the regression of mean crowding on the mean number of adults per plant were not significantly different from 1.0 except for summer populations in 1988. The y-intercepts of these regression equations were not significantly different from 0, suggesting that adults of the potato flea beetle are not aggregated. The ratios of mean crowding to mean number of adults per plant were 0.81 for spring populations and 1.02 for summer populations, also indicating that the populations were not highly aggregated. With a level of precision of 0.25, 10 samples per plot were appropriate when more than 2.8 adults per plant were present in the spring and when more than 7.2 adults per plant were present in the summer.

SEASONAL ABUNDANCE OF FOLIAGE-DWELLING INSECT PESTS IN COMMERCIAL FIELDS AND INSECTICIDE-FREE PLOTS OF POTATO IN MANITOBA

Patterns of seasonal abundance of Colorado potato beetle, Leptinotarsa decemlineata (Say), potato flea beetle, Epitrix cucumeris (Harris), potato leafhopper, Empoasca fabae (Harris), and aphids on potato cv. Norland are reported for the 1983–1985 growing seasons in southern Manitoba. Each year, Colorado potato beetles and potato flea beetles underwent one generation on the crop. There were two peaks of leafhopper abundance in 1983 and 1984, but only one was evident in 1985. Aphid populations rose to a single peak in August of each year.In experimental plots in the absence of insecticides, Colorado potato beetles completely defoliated plants in two seasons out of the three. In commercial fields, insecticide sprays applied in July and early August greatly reduced populations of Colorado potato beetles for the rest of the season, and there was no defoliation at harvest. Insecticides had less effect on potato flea beetle populations. Leafhopper and aphid populations were small at the time of spraying.