MSU Extension Chestnuts

Weekly weather review

Accumulated growing degree days (GDD) base 50 degrees Fahrenheit and accumulated precipitation across the Lower Peninsula are very close to the long-term average, although lower than 2024. Canadian wildfires have created some haze as smoke swirls over the eastern U.S. Most of the smoke has been aloft so air quality is not as bad as it could have been.

Despite average precipitation since March 1, a swath west of Michigan’s Thumb is experiencing moderate-severe drought conditions.

Accumulated GDD50 are around 250 in the Upper Peninsula and approaching 650 in southern Michigan. GDD 50 are near the long-term average across most of state.

Looking ahead

Scattered showers are possible Friday and Saturday, with fair and dry conditions expected Sunday into early next week. Temperatures will be in the 70s this weekend and warmer as the week progresses. Medium range guidance suggests warmer than normal temperatures and above normal precipitation June 16-20, 2025. From June 11-18, 2025 most areas of the state will receive some precipitation. The forecast suggests anywhere from 0.5-1.25. inches depending upon location.

For the most recent Michigan State University agriculture weather forecast, visit: MSU Extension Agriculture Weather.

Management activities

Growers may be soil testing, irrigating, applying fertilizer applications as needed, and scouting for pests. This is also a great time of the year to paint trunks to reduce southwest disease.

Fertility

Most growers using granular fertilizers are planning to apply them soon if they have not already. For nutrient management considerations, please reference the Michigan Chestnut Management Guide or the Nutrient Management section of the MSU Extension Chestnut website. To receive nutrient management recommendations from MSU, pick up a commercial test at your local Extension office.

Integrated pest management

Review the Michigan Chestnut Management Guide and the pest management section of the chestnut webpage for additional pictures and pest management information.

Growers continue reporting black stem borer adults in traps and spraying trunks to protect young trees. Adult black stem borers are very small (0.08 inches long). Black stem borer will infest and damage a wide variety of woody plant species, including chestnuts. Black stem borers are attracted to small trees with less than a 4-inch trunk diameter and stressed trees that produce ethanol. Female borers create tunnels in trunks to lay their eggs. These tunnels damage the tree’s ability to translocate water and nutrients.

Overwintering adults become active in late April or early May after one or two consecutive days of 68 F or higher, often coinciding with blooming forsythia. While emergence peaks in late May and early June, it can continue into late July. Use a simple ethanol baited trap to monitor for activity starting in mid-April. Traps should be placed near wooded areas adjacent to the orchard and at a height of 1.6 feet. Hand sanitizer is an easy and accessible bait but should be refreshed every few days. Traps can consist of just a pop bottle (or similar container) with around a 0.5-1 cup of hand sanitizer.

Growers with small, vulnerable trees and positive trap catches or a history of damage will need to apply a trunk spray to prevent damage. Insecticide treatments target female borer flight in spring, before they colonize trees. Young trees near the perimeter of orchards, especially near woodlots, are at greatest risk of injury. Because they are so tiny, it is impossible to visually scout for adults to determine the optimum time to apply an insecticide, so trapping as described above is recommended to detect adult activity and apply treatment.

Pyrethroid insecticides applied as trunk sprays have shown the most promise in reducing the number of new infestations within a season. For a list of registered pyrethroids for use in Michigan chestnuts, refer to the Michigan Chestnut Management Guide.

Later in the season, remove and burn any damaged dead or dying trees. It is also important to make sure all large pruning debris and brush piles are either burned, or chipped and composted as they may harbor overwintering adults and contribute to future infestations. For more information on black stem borer, refer to the Michigan State University Extension bulletin, “Managing Black Stem Borer in Michigan Tree Fruits.”

Potato leafhopper has arrived in Michigan. Like many plants, chestnuts are sensitive to the saliva of potato leafhopper, 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 brown and yellowed edges and eventually drop from 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 adults. 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.

Scouting for potato leafhopper should be performed weekly as soon as leaf tissue is present to ensure early detection. More frequent spot checks should be done following rainstorms, which carry the first populations north. For every acre of orchard, select five trees to examine and inspect the leaves on three shoots per tree (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 end of branches. For more information on insecticides available for the treatment of potato leafhopper, refer to the Michigan Chestnut Management Guide

Be on the lookout for European red mites. European red mites overwinter as eggs in bark crevices and bud scales and are the most common mite species in Michigan chestnut orchards. Eggs are small spheres, about the size of the head of a pin with a single stipe or hair that protrudes from the top (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.

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 in color and have hairs that give them a spikey appearance. Adult and nymph feeding occurs primarily on the upper surface of the leaves. This first generation is the slowest of the season to mature and reproduce, typically taking a full three weeks. This slow development is due to the direct link between temperature and mite development. Summer generations, favored by the hot and dry weather, can complete their lifecycles much faster with as little as 10 days between generations under ideal conditions.

As you scout, 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. Predaceous mites are smaller than adult European red mites and twospotted spider mites, but they can be seen with a hand lens and typically move very quickly across leaf surfaces.

Mite control starts with monitoring early in the spring looking for the overwintering eggs (European red mites) and assessing the mite pressure. Ideally, growers will be using limited insecticides with miticidal activity in their season long programs, as that protects their beneficial mite populations which help minimize pest mites. If pest mite populations are high enough to require control, superior oil application when the trees are dormant is an effective method of treatment. If issues with mites arise during the growing season, refer to the Michigan Chestnut Management Guide for control options.

Looking ahead, growers with Asian chestnut gall wasp populations in the orchard and those adjacent to known infestations should start scouting as they approach 1,050 GDD50. To read up on gall wasp, review the bulletin "Asian Chestnut Gall Wasp: An invasive pest of concern for Michigan chestnut growers."

Disease

New research out of Michigan State University has provided new insight into chestnut brown rot management. Growers planning to manage brown rot this season should carefully review “Chestnut brown rot management in the field” in preparation for a bloom-time preventative fungicide program.

Existing chestnut blight infections (caused by Cryphonectria parasitica) can be observed at this time. There are no commercially available treatments for chestnut blight. Growers may prune out infected branches or cull whole trees as needed to limit disease pressure. Infested material should be burned or buried to further limit inoculum spread. To learn more about chestnut blight, visit the pest management section of the chestnut webpage.

Stay connected

For more information on chestnut production, visit www.chestnuts.msu.edu. Sign up to receive our newsletter and join us for the 2025 MSU Chestnut Growers Chat Series.

If you are unsure what is causing symptoms in the field, you can submit a sample to MSU Plant & Pest Diagnostics. Visit their webpage for specific information about how to collect, package, ship and image plant samples for diagnosis. If you have any doubt about what or how to collect a good sample, please contact the lab at 517-432-0988 or pestid@msu.edu.

Become a licensed pesticide applicator

All growers utilizing pesticide can benefit from getting their certification, even if not legally required. Understanding pesticides and the associated regulations can help growers protect themselves, others, and the environment. Michigan Pesticide Applicator Certification is administered by the Michigan Department of Agriculture and Rural Development. You can read all about the process by visiting the Pesticide FAQ webpage. Michigan State University offers a number of resources to assist people pursuing their license, including an online study/continuing ed course and study manuals.

This work is supported by the Crop Protection and Pest Management Program [grant no 2024-70006-43569] 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.