Chestnut scouting report - June 3, 2019
Growers should be scouting for potato leafhopper, which has arrived in the state.
Chestnut trees have finally pushed out some leaves up north after a slow start this spring with unusually cool temperatures. As leaves expand, winter injury is becoming evident in a number of orchards, including in established orchards with large trees. Many growers experienced heavy crop loads last year that limited terminal branch growth, which is an indicator of tree stress. Tree stress and cold, windy winter conditions contributed to damage. Additionally, trees with existing chestnut blight infections appear disproportionately affected. In some cases, growers are observing complete tree death, but more commonly main branches and terminal growth are affected.
Potato leafhopper is being reported around the state at this time and growers are encouraged to be actively scouting. Like many plants, chestnuts are sensitive to the saliva of potato leafhopper (PLH) which is injected by the insect while feeding. Damage to leaf tissue can cause reduced photosynthesis which can impact production and quality, and damage the tree. Most injury occurs on new tissue on shoot terminals with potato leafhopper feeding near the edges of the leaves using piercing-sucking mouthparts. Symptoms of feeding appear as whitish dots arranged in triangular shapes near the edges. Heavily damaged leaves are cupped with yellowing, dying edges and eventually fall off the tree. Severely infested shoots produce small, bunched leaves with reduced photosynthetic capacity.
Adult leafhoppers are pale to bright green and about 1/8-inch long. Adults are easily noticeable, jumping, flying or running when agitated. The nymphs (immature leafhoppers), are pale green and have no wings but are very similar in form to the adults. PLH move in all directions when disturbed, unlike some leafhoppers which have a distinct pattern of movement. The potato leafhopper can’t survive Michigan’s winter and survives in the Gulf States until adults migrate north in the spring on storm systems.
Perform scouting weekly as soon as leaf tissue is present to ensure detection early and prevent injury. More frequent spot checks should be done following rain storms, which carry the first populations north. For every acre of orchard, select 5 trees to examine and inspect the leaves on 3 shoots per tree (a total of 15 shoots per acre). The easiest way to observe potato leafhopper is by flipping the shoots or leaves over and looking for adults and nymphs on the underside of leaves. Pay special attention to succulent new leaves on the terminals of branches. Most growers utilize foliar imidacloprid application for control as it is highly effective, less toxic to beneficials and has a longer residual activity. For more information on insecticides available for the treatment of potato leafhopper refer to the current Chestnut Management Guide.
Rose chafer emergence should begin over the coming weeks. Rose chafers (RC) are considered a generalist pest and affect many crops, particularly those found on or near sandy soils or grassy areas. The adult beetles feed heavily on foliage and blossom parts of numerous horticultural crops in Michigan and can cause significant damage to chestnut orchards. Rose chafer can be particularly damaging on young trees with limited leaf area. Rose chafer skeletonize the chestnut leaves, but tend to consume larger pockets of tissue, with damage similar to caterpillar feeding rather than the fine lace-like leaf that results from Japanese beetle feeding.
They are often found in mating pairs and fly during daylight hours. Visual observation while walking a transect is the best method for locating them. Because of their aggregating behavior, they tend to be found in larger groups and are typically relatively easy to spot. There are no established treatment thresholds or data on how much damage a healthy chestnut tree can sustain from rose chafer, but growers should consider that well-established and vigorous orchards will likely not require complete control. Younger orchards with limited leaf area will need to be managed more aggressively.
The rose chafer is a light tan beetle with a darker brown head and long legs and is about twelve millimeters long. There is one generation per year. Adults emerge from the ground during late May or June and live for three to four weeks. Females lay groups of eggs just below the surface in grassy areas of sandy, well-drained soils. The larvae (grubs) spend the winter underground, move up in the soil to feed on grass roots and then pupate in the spring. A few weeks later, they emerge from the soil and disperse by flight. Male beetles are attracted to females and congregate on plants to mate and feed. For more information on insecticides available for the treatment of rose chafer refer to the current Chestnut Management Guide.
Chestnut trees are also susceptible to feeding damage from a number of spider mite species including European red mite and two-spotted spider mites. Affected leaves appear mottled, stippled, or bronzed and become brittle leading to early defoliation and reduced photosynthetic activity. Reduced photosynthesis can lead to reduced nut size and return crop load in subsequent years as well as increased sensitivity to winter injury. At this time no treatment thresholds are established for mites in chestnut, but evidence from crops like cherry indicate that some level of feeding is likely tolerable and that higher populations can be tolerated as the season progresses through summer. European red mite appears to be the more prevalent mite species for chestnut producers, but growers should keep an eye out for two-spotted spider mites as well.
European red mites (ERM) overwinter as eggs in bark crevices and bud scales. Eggs are small spheres, about the size of the head of a pin with a single stipe or hair that protrudes from the top however, this is not always visible. Eggs can be viewed with a hand lens or the naked eye once you have established what you are looking for.
Growers can scout for overwintering eggs and early nymph activity in the spring to assess population levels in the coming season. As temperatures warm, overwintering eggs hatch and nymphs move onto the emerging leaves and start feeding. Adult European red mites are red and have hairs that give them a spikey appearance. Adult and nymph feeding occurs primarily on upper leaf surfaces. This first generation is the slowest of the season and typically takes a full three weeks to develop and reproduce. This slow development is due to the direct link between temperature and mite development. Summer generations, favored by the hot and dry weather are able to complete their lifecycles much faster with as little as ten days between generations under ideal conditions.
Growers should also be looking for two-spotted spider mites (TSSM) when scouting. These mites have two distinct spots located on the front half of the body. Males are much smaller than females, and have a distinctly pointed abdomen. Two-spotted spider mite eggs are spherical and translucent and are often found along leaf veins. The nymphs and adults can be a variety of colors depending on the time of year and development stage. Potential colors include orange, brown, pale yellow or green so growers are encouraged to use the spots as the key identification characteristic.
Two-spotted spider mites overwinter as adults around the base of trees in the duff. As the weed or grass cover under the trees dries out, the mites start to move back up into the canopy in higher numbers from tight cluster through harvest. Like European red mites, two-spotted spider mites damage the tree by feeding on the underside of the foliage and give the leaves a dirty appearance when populations become high and leaf surfaces are coated in eggs, discarded exoskeletons and webbing.
As growers are scouting, they are encouraged to remember that not all mites are bad! Consider documenting the levels of predacious mites in your orchard. If healthy populations of mite predators exist, they will continue to feed on plant parasitic eggs and nymphs and can be an effective component of your mite management program. The three most important predaceous mites are Amblyseius fallacis (Phytoseiidae), Agistemus fleschneri (Stigmaeidae), and Zetzellia mali (Stigmaeidae). Predaceous mites are typically translucent, they can be seen with a hand lens and typically move very quickly across leaf surfaces compared to pest mites.
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June 7-9, 2019 Chestnut Growers of America Annual Meeting in Michigan at the Kellogg Biological Station in Hickory Corners.
September 7, 2019 Midwest Chestnut Producers Council Farm Tour at Nash Nurseries in Owosso, MI. Time and Agenda TBA later.
This work is supported by Project GREEEN and the Crop Protection and Pest Management Program 2017-70006-27175 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
This work also supported by the Rogers Reserve Endowment.