Potato psyllid vector of zebra chip disease in the Pacific Northwest

Zebra chip (ZC) is a destructive disease of potatoes in North America and other parts of the world. It has been very costly to manage in potato crops and has caused millions of dollars in losses to the potato industry in the southwestern United States, particularly Texas.

ZC is transmitted by the potato psyllid, which is the only known vector in potato (Figure 1). The disease was first recorded in Idaho and the Columbia Basin of Washington and Oregon late in the 2011 growing season. This region produces more than 56% of the potatoes grown in the United States, so the presence of ZC has the potential to be economically devastating.

Since 2012, the incidence of ZC has been relatively low even though the abundance of the vector has fluctuated (Figure 2). However, because it could have strong negative effects on the North American potato industry, a comprehensive plan is needed to tackle the vector and the pathogen.

Despite the progress made to better understand the ZC vector and pathogen, further information is needed to fully manage this complex insect-pathogen system. This publication provides basic up-to-date information regarding this pest complex.

Brief history and distribution of ZC

ZC was first documented in potato fields near Saltillo, Mexico, in 1994. In the early 2000s, the disease was reported in southern Texas, and by 2006, ZC had spread to all potato production areas in Texas. Since then, ZC has been found in Arizona, California, Colorado, Kansas, Nebraska, Nevada, New Mexico, Wyoming, Oregon, Washington, Idaho and Utah. ZC is also found in Guatemala, Honduras, Mexico, New Zealand, Nicaragua and Australia (Figure 3).

To date, four haplotypes (distinct genetic types of the potato psyllid that are associated with different geographical regions) of the potato psyllid have been identified; they are the central, western, northwestern and southwestern haplotypes. In Oregon and Washington, the northwestern and western haplotypes are predominant, while in Idaho, all haplotypes are present.

A related psyllid, B. maculipennis, occurs in high numbers on bindweed and is sometimes captured on traps in potato fields. This psyllid has black markings on the wings (which are not present on potato psyllid) and is not a threat to commercial crops (Figure 4).

Eggs

The football-shaped eggs (Figure 5) are extremely small, just slightly larger than potato leaf hairs. Eggs are yellow-orange and are attached individually to leaves on a short stalk. They are usually laid in the plant canopy on the underside and along the edges of leaves. A hand lens with at least 10× magnification is required to see them. Eggs hatch in six to 10 days, depending on temperature. Warmer temperatures (78° F to 82° F or 26° C to 28° C) favor early hatching, although temperatures above 90° F (32° C) negatively impact reproduction and survival.

Adults

Psyllid adults are about 0.08 inch (2 millimeter) long and have clear wings that are held roof-like over the body when at rest. They are closely related to aphids and leafhoppers and resemble small cicadas, winged aphids, or bark lice.

The potato psyllid is predominantly black with white markings (Figure 1). The first abdominal segment shows a broad white band and the last segment has an inverted white “V.” Newly molted adults are yellowish green (Figure 7), and are, therefore, difficult to distinguish from other insects. Adult psyllids jump readily when disturbed. An adult female can lay over 1,000 eggs during her lifetime and can pass the ZC pathogen directly to her offspring.

Overwintering

Potato psyllid has long been known to be present throughout the Pacific Northwest. It was thought that psyllids were unable to overwinter in the area and that reintroduction in any given season required migration on air currents from the southwest part of the United States, possibly California. However, observations made during the winter and spring confirm that adult psyllids can survive the winter in certain areas of the PNW (Figure 8).

Psyllid have been found on S. dulcamara L., commonly known as bittersweet nightshade, bitter nightshade, blue bindweed or poison berry. This perennial weed, native to Eurasia, is found in wet and dry fencerows, pond margins, low woods and roadsides throughout the PNW. The psyllid also overwinters on the perennial shrub matrimony vine (Lycium spp., also called goji berry or wolfberry) (Figure 9). The shrub is well adapted to arid/semiarid climates and grows in dense patches that are often close to historic homesteads or cemeteries. Research confirms that not only can potato psyllids overwinter in the PNW but that a small percentage of the overwintering weeds can harbor the ZC bacterium.

Direct feeding damage

Psyllid nymphs and adults feed similarly to aphids, probing host plants with their needle-like mouthparts and sucking plant juices (Figure 10). Because they feed directly on the plant, potato psyllids can cause plant damage even when not carrying the bacterium responsible for ZC. This phenomenon was observed in recent years when ZC bacterium was almost nonexistent in psyllids tested.

As they feed, psyllids inject toxins with their saliva that can cause leaf yellowing or purpling, smaller and fewer tubers, and misshapen tubers. This physiological condition is called “psyllid yellows” (figure 11) and is generally less damaging than ZC, as it does not cause the characteristic ZC symptoms seen in the cut tuber or following frying.

Damage from the ZC pathogen

ZC usually requires about three weeks following pathogen inoculation by psyllids to produce symptoms in the foliage and tubers. Plants infected by ZC exhibit a range of aboveground symptoms that are similar to potato purple top and psyllid yellows, including stunting, chlorosis, leaf scorching, swollen internodes near apical portions, axillary bud and aerial tuber proliferation, necrosis of vascular system and early death (Figure 12). Symptoms in tubers include development of dark striped patterns of necrosis (Figure 13). The disease also alters the starch metabolism of infected tubers, converting starch into sugars in zones of the tuber.

Abundance

In 2011–2012, when ZC was first reported in the PNW, psyllid adults were first detected in potatoes in early July. However, in recent years, adults have been detected in the upper Columbia Basin and Idaho in the last week of May and the first week of June. In the lower Columbia Basin, psyllids have been found the last week of May (Figure 2).

In all geographical locations, potato psyllid adult numbers are relatively low during early to mid-season, but by late season, abundance can peak dramatically. It has been suggested that one reason for high numbers is the rapid movement of adult psyllids to nearby fields following the harvest of early planted potatoes.

Sampling

A sampling program to monitor psyllid populations is a fundamental tool for a successful integrated pest management program. Sampling programs are critical for decision-making strategies.

Yellow sticky cards

Unbaited 3 inch by 5 inch (8 centimeter by 13 centimeter) yellow sticky cards are recommended to detect the first occurrence of psyllids in the area (Figure 15), though their sensitivity to confirm psyllids at low populations (i.e., early in the season) may be low.

Attach cards to 28- to 30-inch (70- to 75-centimeter) wooden stakes and place them at canopy level; adjust the cards to plant canopy height by moving them along the stake.

Start seasonal sampling as soon as the potato season begins. Replace sticky cards weekly. Sticky traps will likely be most useful for detecting psyllid migration into and out of fields.

Spatial and temporal studies indicate that early in the season both psyllid abundance and ZC incidence are higher in field edges than in the interior of fields.

Sweep net and aspirator or vacuum sampling device

Considering that adult potato psyllids are active and fly or jump away when disturbed, a sweep net plus an aspirator or a vacuum sampling device such as a D-Vac or an inverted leaf blower can also be used to monitor psyllids (Figure 16). Texas A&M University recommends 100 sweeps from around the field perimeter.

If you use a vacuum sampler, be sure to use it 5–10 feet (152.4–304.8 centimeters) inside the field edge for at least five minutes. It is more difficult to sort psyllids collected by these methods than to count them on a sticky card. For more information on using a sweep net, see the video “Sweep Net Technique."

Leaf sampling

Collect 10 leaves from 10 locations among the 10 outer rows of the field (Figure 17). Collect full-size leaves from the middle of the plant to look for psyllid eggs or nymphs. You may need a hand lens to see these life stages. The nymphs usually reside on the underside of the leaf. Eggs are most commonly present on the leaf’s edges and underside; however, if the psyllid population is high, eggs can be found on other parts of the leaves.

Keep in mind that this is not the preferred method for determining the first occurrence of these insects. Leaf sampling confirms that psyllids are already colonizing the field, and ZC infection will likely have happened by then if the psyllids are carriers of the bacterium.

Action threshold

No action threshold exists for psyllids in potato. However, we suggest that the threshold for action is first detection of potato psyllids at any level, in any life stage. Psyllids can cause direct damage by feeding and as a vector of the ZC pathogen.

Chemical control

There are a number of insecticides registered on potatoes that have activity against potato psyllids in the adult stage, or immature stage, or both. Some insecticides with activity against adults and nymphs will also have activity against the eggs.

Season-long, weekly applications are used in areas where ZC has been problematic (e.g., in Texas, psyllids are present from planting to harvest). In the PNW, especially in areas that have had problems with ZC, the recommendation is to start chemical treatments as early as mid-May, since in the last several years adult psyllids were detected earlier (Figure 2).