Damage: Aphids can cause premature senescence and reduce yields, especially large populations. But the main economic impact of aphids on potatoes is virus transmission, especially Potato Leafroll Virus (PLRV) and Potato Virus Y (PVY). Green peach aphid and potato aphid are the most common aphid species in potato fields, but other aphids can spread viruses while passing through in search of more suitable hosts.
Scouting: Potato growers should monitor fields for aphids often to identify early infestations. Aphids can reproduce rapidly and large infestations can be difficult to control with a single insecticide application.
Control: Some aphicides are better than others at limiting spread of PVY. Many that work via ingestion kill aphids too slowly to prevent spread; an exception is selective feeding blockers that kill slowly but stop feeding within hours. Contact insecticides can be useful to knockback aphid populations, but many do not have long enough residual activity or coverage of new growth to prevent migrating aphids from spreading PVY between applications.
Additional Details »The bertha armyworm, spotted cutworm, and cabbage looper are the most important caterpillars of Lepidopteran moths found in potatoes in the Pacific Northwest. They are pests because they feed on potato leaves. Infestations usually start in early summer.
Scouting: Loopers may be found feeding on the leaves during the day. Other species are nocturnal and are more difficult to scout, but they may be found by lifting up the plants and looking at the soil surface. Caterpillars get larger as they progress through several larval instars; growing up to 1.5 to 2 inches long.
Damage: Studies have shown that potato plants can tolerate at least 10% defoliation by these caterpillars with no marketable yield loss. They are slower to buildup populations compared to Colorado potato beetles, and are much slower to defoliate plants. Early infestations are more important (more destructive) than late infestations.
Additional Details »Damage: Beet leafhoppers (BLHs) are important pests of potato because they can transmit a phytoplasma (BLTVA) that causes purple top disease. Since BLHs are transient, or pass-through pests of potatoes, they may be gone by the time you see purple top symptoms. So, it is important to monitor BLHs as their populations develop and move through the region.
Control: BLH numbers usually peak in June. Insecticides targeting BLHs are usually applied May to June, and sometimes July. They are not usually warranted later in the season regardless of BLH numbers.
Additional Details »Colorado potato beetles (CPBs) overwinter as adult beetles in the soil (about 8-16 inches deep) in the previous year's potato fields. They emerge in the spring to feed, mate, and lay eggs (up to 1,000 eggs per female in masses of 20-50 eggs). Larvae hatch from eggs, feed and grow through four larval instars, and then drop to the ground to pupate and start the cycle over again. They can have up to three generations in a year.
Damage: Adults and larvae can cause complete defoliation and nearly complete crop loss if allowed to reproduce unchecked. Larvae are more voracious than adults.
Scouting: CPBs are easy to scout. Look for them on plants near the edge of potato fields, and on volunteer potatoes in other crops (volunteer potatoes can serve as important refuge for CPBs and a source of delayed infestations). Adult beetles are about 0.5-inches long, with an orange-and-black head and yellow-and-black striped wings. Once you find adults, look for egg masses. Their oval eggs are yellow-to-orange in color. Scout often for larvae once you find eggs. Newly hatched larvae have black heads and reddish-orange abdomens. As they age, they get bigger and lighter in color with distinct black dots on their sides.
Additional Details »Damage: When lygus bugs feed, they damage plant cells with their piercing mouth-parts. This can result in significant injury to foliage, especially when feeding begins early and damage accumulates over the season. Symptom distribution across a field can be extensive, and is not restricted to border areas like many other insect-related symptoms.
Additional Details »Potato psyllids are important potato pests because they can spread a bacterium that causes zebra chip disease. Zebra chip reduces yields by causing premature vine death, and quality by causing an internal tuber browning defect.
Click HERE to read an article that describes three strategies that are used by growers in the Columbia Basin to control potato psyllids using insecticides.
Additional Details »Damage: The larval stage of the potato tuberworm (the adult stage is a moth) can damage potato tubers in the weeks leading up to harvest, especially after vine kill or vine senescence, when larvae are forced to move from the vines to tubers. The larvae feed on tubers causing tunnels.
Scouting: Potato tuberworms are easy to monitor using pheromone traps. The pheromone lure used inside the sticky trap attracts male moths that are looking for females. Click HERE for a guide to deploying pheromone traps.
Additional Details »Scouting: Spider mites are easy to monitor despite their small size. Shake plants vigorously over a pail or bucket and then look for the tiny moving dots; they look like specs of sand walking around in the bottom of your pail. Initial infestations can be spotty, so it is important to check several places near the edge of the field. Spider mites are often found in potatoes near dusty roads or mite-harboring crops like alfalfa, corn, mint, or wheat.
Control: In many cases, a single, well-timed application will control spider mites. If you can see webbing on the leaves and bronzing or stippling from mite feeding, then it may be too late to get effective control with one application. Most miticide products target eggs and immature stages.
Additional Details »Fusarium dry rot is a tuber rotting disease that can be caused by several Fusarium species., including F. sambucinum, F. coeruleum, and F. avenaceum. Tuber infection does not usually occur unless the tuber is injured during harvest. Wounds provide a way for the fungus to enter the tuber. Fusarium dry rot can lead to secondary infections by soft rot bacteria.
Additional Details »Rhizoctonia canker and black scurf are caused by the fungus, Rhizoctonia solani. The fungus can be spread from the soil, or infected seed and seed pieces.
The fungus can cause reddish-to-dark brown lesions or cankers on sprouts, roots, stolons, and underground stems. Sprouts can be girdled by cankers, and killed before they emerge. Girdling cankers on stolons can pinch off small tubers. Stems seem to become more resistant to infection after emergence. But severe infections can result in cankers on above-ground stems. Severe stem cankers can disrupt water movement in the plant, resulting in yellow leaves and wilting.
Black scurf (fungal masses) can form on the surface of mature tubers. The scurf is not easy to remove, and tubers with a lot of scurf may be rejected for the fresh market.
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Silver scurf is caused by the fungus, Helminthosporium solani. It is spread primarily by infected seed.
Additional Details »Root-lesion nematodes (RLNs) are migratory endoparasites (Pratylenchus species) that move freely in and out of roots.
Damage: These nematodes don’t blemish tubers, but they can make plants less vigorous and reduce yields. Damage is directly related to the number of nematodes present, and potatoes can tolerate low numbers without significant issues.
Control: The recommended treatment threshold often depends on the species; it can be as low as 50 nematodes per 100 cc soil for P. penetrans, or as much as 500 nematodes per 100 cc soil for the less aggressive P. neglectus. Pratylenchus penetrans is one of the more destructive species, and it is known to interact with the soilborne fungus, Verticillium dahliae, to cause and increase the severity of early dying disease. Aggressive management of the nematode is recommended when both P. penetrans and V. dahliae are found in the soil, so the treatment threshold may be set as low as 25 nematodes per 100 cc soil.
Additional Details »The most important species of root-knot nematode (RKN) on potatoes are 1) northern root-knot nematodes (M. hapla) that cause small root galls and random tuber swellings; 2) southern root-knot nematodes (M. incognita) that cause large root galls and warty bumps on tubers; and 3) Columbia root-knot nematodes (M. chitwoodi) that cause pimple-like bumps on tubers.
Damage: RKNs are endoparasites that live mostly inside plants. They are important potato pests because of the galls they produce on roots and the tiny necrotic spots they cause under the tuber skin. A female nematode and her mass of eggs can be found in each spot.
Control: Treatment with soil fumigants, biofumigants, or other chemical controls is recommended when any root-knot nematodes are found in soil samples, because it doesn’t take many of them at the beginning of the season to end up with significant damage at harvest.
Additional Details »Stubby-root nematodes are ectoparasites that live entirely in the soil and feed on root tips.
Damage: They are important pests of potato because they transmit Tobacco rattle virus (TRV), which is the causal agent of corky ringspot disease. Symptoms of corky ringspot include internal brown arcs, rings, or blotches inside the tuber, and arc-shaped lesions on the tuber surface. Stubby-root nematodes rarely damage potatoes without the virus, but they can cause significant stunting of more susceptible plants like onions.
Control: The treatment threshold is upon detection of the nematode when corky ringspot is a concern, but it is often set at 200 or more nematodes per 100 cc soil in fields that are not infested with TRV.
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