What's wrong with my potato tubers?

Diagnosing tuber abnormalities in Western Oregon and Washington

Organism: Several species of flea beetles can be found on potato vines, but the tuber flea beetle (Epitrix tuberis) is the most common species that damages tubers in the Pacific Northwest (Figure 1). Flea beetle adults are about ¹/₁₆ inches long and jump relatively large distances when disturbed. Tuber flea beetles are common pests of potatoes and other Solanaceous crops in western Oregon.

Symptoms: Adults feed on the foliage, giving leaves a characteristic shot-hole appearance (Figure 2). Eggs are laid near plant stems, and larvae feed on tubers. Flea beetle larvae generally cause thin tracks on the surface of tubers and do not burrow deep into the flesh (Figure 3). Damage often is worse at the edge of a field since adults migrate in from field margins. Damage also progresses during the season as larvae grow and feeding intensifies.

Organism: Wireworms are the larval stage of “click” beetles in the family Elateridae. Several species can damage potato tubers in Western Oregon. Limonius species, the Pacific Coast wireworm (L. californicus) and the sugar beet wireworm (L. canus), are often important in this area.

In 2006, invasive European wireworm species (Agriotes spp.) were found at several sites in the Portland area. These introduced species have been moving south from British Columbia and may become more important over time. Adult “click” beetles have a slender, black body about ¹/₃ to ³/₄ inches long (8–20 mm) (Figure 4). Larvae are about ¹/₁₆ inches long after hatching and 1¹/₂ inches long or more at maturity (2 mm– 4 cm). They are slender and shiny yellow to light brown with a hard cuticle and three pairs of small legs near the head (Figure 5). Larvae live from two to five years in the soil.

Organism: Early blight is caused by the fungus Alternaria solani.

Symptoms:

• Foliar: Early blight often develops on aging or nitrogendeficient vines. On leaves and stems, the lesions first appear as small black or brown spots 1 to 2 mm in diameter (Figure 8). As lesions develop, they form concentric black and brown rings (“target spot”) with yellow halos. The lesions typically are bordered by leaf veins (Figure 9).
• Tuber: Tuber damage is less common than leaf infection, but can develop over time in storage. Tuber lesions begin as sunken brown or black spots that penetrate the flesh of the tuber, developing into dry, brown, corky tissue (Figures 10–12).

More information: Michigan State University Extension Bulletin E-2991.

Organism: Black scurf is caused by the fungus Rhizoctonia solani.

Symptoms:

• Tuber: Black scurf appears as brown or black, durable, raised masses on the surface of the tuber (Figure 18). These masses are the resting structures of the pathogen, called “sclerotia.” The sclerotia may look like small clumps of soil, but, unlike soil, they are very difficult to remove. Rhizoctonia often is described as “the dirt that won’t wash off.”
• Foliar: This pathogen causes another disease, Rhizoctonia stem canker, which affects potato sprouts as they begin to grow from the seed piece through the soil. For diagnostics and management of this disease, see Rhizoctonia Stem Canker and Black Scurf of Potato. Michigan State University Extension bulletin E-2994.

Hollow heart (Figure 25, with growth cracks) is a discolored lens- or star-shaped cavity found in the center of an otherwise healthy tuber.

Cause: Hollow heart is associated with rapid tuber growth after cool temperatures and moisture stress (too much or too little water). Large tubers usually, but not always, are more susceptible than small tubers.

A rough, irregular surface of the tuber skin is referred to as elephant or alligator hide (Figure 26). The disorder is shallow and appears scaly or crackled.

Cause: The primary cause of this condition is unknown, but contributing factors may include high temperature, variety, high soil organic matter content, and excessive soil moisture and fertilization. Some researchers speculate that elephant hide is caused by uneven growth of the tuber.

Lenticels are openings (ruptured stomates from tuber skin expansion) on the potato that allow for air exchange. They can become enlarged when exposed to waterlogged soils or prolonged wet conditions. Enlarged lenticels look like small, white bumps on the surface of the tuber (Figure 27).

Cause: Oxygen availability drops in saturated soils or when tuber surfaces remain wet for extended periods of time in storage; in response, lenticels open and become enlarged. This disorder is not merely superficial, as it increases susceptibility to the entry of disease organisms, especially soft rot.

A green hue on the tuber surface is referred to as “greening” and indicates the accumulation of chlorophyll (Figure 28).

Cause: Potato tuber exposure to light in the field causes the formation of a green pigmentation on the potato. This occurs when sunlight directly contacts tubers growing at or near the soil surface or reaches tubers through cracks in the soil surface. Greening typically affects a limited part of the tuber. The same effect can be caused by extended exposure to low light levels in storage or on store shelves. In this case, the result typically is a lighter, more diffuse coloration on the entire tuber.