Garlic IPM Newsletter No. 4 — May 21, 2026

Warmer weather conditions over the last week have encouraged good garlic growth at all scouting locations, and plants will soon begin bulbing. Now is the time to apply the final nitrogen sidedress of 40 lb N per acre (or about 0.09 lb N per 100 sq ft). This will support the last of the plants’ foliar growth before they shift to bulb growth. Do not apply nitrogen after early June, as this will encourage top growth at the expense of bulb development.

Disease Monitoring

No insect pests have been observed at scouting sites, but a few foliar diseases have made an appearance.

Botrytis leaf blight (BLB) lesions in garlic begin as distinct necrotic spots with slight yellow halos that enlarge with disease development.

Purple blotch (PB) symptoms appear as a few necrotic spots with more general background chlorosis, and coalesce into elongated necrotic spots that later turn brown or purple with a distinctive bullseye pattern.

While symptoms of these two fungal pathogens appear similar, notice that the BLB lesions are more distinct and the background yellowing of PB lesions is more extensive. If you notice symptoms like these on your garlic (or other allium) crop, consider submitting leaf samples to the UMaine Plant Disease Diagnostic Lab, which can identify the causal pathogens.

BLB is more problematic in onion than garlic, and no action threshold has been developed for this disease in garlic specifically. In onion, BLB can severely infect foliage and cause premature leaf dieback, resulting in undersized bulbs and necks that are too brittle and diseased to lodge properly at harvest. This, in turn, often leads to poor storage. The same is true of PB, with a key difference being that PB infections in garlic can also be severe enough that yield may be reduced. Both pathogens can spread between garlic and onion plantings, which can be an important consideration when deciding whether or when to actively manage these diseases.

Of course, management decisions should be based on whether conditions are favorable for disease development and spread, in addition to disease presence. This is when forecasting tools like NEWA come in handy. The onion diseases tool lumps together BLB, PB, and onion downy mildew since fungicide programs typically consider disease pressure for all three.

The “More Info” button below the results table has information about each pathogen and how to use the forecast to support your management decisions. Below is a quick distillation for each disease. Keep in mind, however, that these recommendations are for onion, as no thresholds have been developed for garlic.

• Botrytis leaf blight: The Modified Blight Alert (IPI) is the most relevant BLB column to use currently (the Michigan model predicts early season spore release, and BLB has already been confirmed). The action threshold for the first fungicide application is when a) there is at least one lesion per leaf on average, b) the IPI > 7, and c) the chance of rain is 30% or greater.
• Onion downy mildew: When 5 out of 7 days in a given timeframe are “favorable” for this pathogen, and where DM is known to have been a problem in the past, maintain excellent protective fungicide coverage.
• Purple blotch: This model is still being developed, but a PRI > 5.7 indicates greater risk of infection. Plants are typically most susceptible while bulbing.

No Maine sites on NEWA show any risk of onion disease development through May 26. Still, it may be helpful to review what control materials are available based on what problems you may have had in the past. The New England Vegetable Management guide lists products specific to garlic diseases, and you can also look at the conventional, biorational, and biological fungicide tables for products. As with any pesticide, the label is the law. Any product used must be labelled specifically for that crop and registered for use in Maine (or your state).

Using a 3-4 year crop rotation, avoiding overhead irrigation, removing crop residue, and encouraging good airflow (with planting density and direction) are the key cultural management practices that can reduce the frequency and severity of these foliar diseases.

Onion Thrips

These insects are the most reliable pests of allium crops in Maine, and they’re likely to emerge in southern Maine within the next week or so, at 140 GDD52.7°F. You can track this on the Maine Climate Office GDD map; be sure to look at the base 52.7°F map at the bottom-right of the page. More on these pests next week.

 

Peyton Ginakes, PhD
Research Associate
University of Maine Cooperative Extension
Highmoor Farm
52 US-202
Monmouth, ME 04259
(207) 933-2100

peyton.ginakes@maine.edu

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This project is funded by a Specialty Crop Block Grant through the Maine Department of Agriculture, Conservation, and Forestry. Funding for the Maine 2024 Specialty Crop Block Grant Program was made possible by a grant/cooperative agreement from the USDA Agricultural Marketing Service. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the USDA.

Where brand names or company names are used, it is for the reader’s information. No endorsement is implied nor is any discrimination intended against other products with similar ingredients. Always consult product labels for rates, application instructions and safety precautions. Users of these products assume all associated risks.

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