Garlic IPM Newsletter No. 5 — May 15, 2025

Potential Botrytis Leaf Blight

Symptoms of Botrytis leaf blight were found on garlic at all scouting locations this week – Buxton, Monmouth, Topsham, and Washington, ME. Samples of symptomatic plants are being sent to the UMaine Plant Diagnostic Lab for confirmation. Conditions certainly have been favorable for development of this pathogen, as discussed in last week’s newsletter.

Botrytis leaf blight/blast (BLB) is caused by Botrytis squamosa – not the B. cinerea commonly known as gray mold in many other crops, or the other species that cause Botrytis neck/bulb rot. The first symptoms to appear are small halo lesions, followed by necrotic spots that appear bleached. Follow this link for an image of an onion leaf showing necrotic lesions caused by BLB.

Lesions resulting from this disease continue to develop in humid or wet weather. As it does so, leaf tips and entire leaves may die back, resulting in stunted plants and reduced yields. This pathogen can spread quickly in ideal conditions, hence the alternative name leaf “blast”.

If you look to the New England Vegetable Management Guide for control options, note that this specific pathogen is detailed in the onion section, but not the garlic section. Those cultural controls listed for onion apply to garlic. Fungicide options include those found in the onion BLB section (they MUST also be labeled for garlic) and those for other fungal diseases of garlic (check labels to see if they claim to control BLB specifically). Refer back to the Week 4 newsletter for action thresholds and using NEWA. Per NEWA, weather conditions in Lewiston favor both onion downy mildew infection and continued development of BLB; Milton, Rochester, and Shelburne, NH are also at risk of BLB development this weekend.

University of Maine Plant Diagnostic Lab

Positively identifying problems is critical for determining how to manage them. If you are unsure about symptoms you see on garlic, or any plants, you can work with UMaine’s Diagnostic Lab to verify the pathogen(s). For garlic, recommended options for diagnosis involve submitting either photos or plant samples.

• Digital Diagnosis: Enter your information and upload photos to the online form. Each submission costs $8.
• Physical Diagnosis: Carefully follow instructions on the form. Each sample costs $18 for Maine residents ($38 for out-of-state). Send samples soon after collection and toward the beginning of the week. Note that garlic seed testing (separate from the general test) is geared toward growers wishing to sell certified clean seed stock, although it may be useful for anyone saving their own seed.

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Insect Pest Situation

Update! The Maine Climate Office GDD maps now include relevant base models for more allium pests – the base 33.8°F for allium leafminer and base 52.7°F for onion thrips.

The only insect pests seen on garlic at scouting locations this week were aphids, despite sufficient GDD accumulations for some other pests. The table below shows where pests may become problematic in the near future.

Onion Maggot: The overwintering generation begins to emerge at 390 GDD_40°F, which has been reached in Damariscotta, Fryeburg, Kittery, and Sanford. Many other locations – north to Newry, Dover-Foxcroft, and Ellsworth – are approaching this threshold. If you use physical barriers like row covers to protect alliums, they must be in place by at least peak flight (735 GDDs), but ideally before this 390 GDD emergence threshold.

Onion Thrips: Thrips prefer hot, dry weather and typically begin to show up in alliums in early June. This year is on track for average egg hatch timing, which occurs at 140 GDD_52.7°F, as all Maine locations remain below 100 GDDs.

Allium Leafminer: ALM is not shown in the table below because adults emerge at 350 GDD_33.8°F, which has been far surpassed in all Maine locations except Katahdin. That said, we have yet to detect this pest in the state despite its spread through New York, Pennsylvania, and Massachusetts. If you see the telltale marks left by ALM on your allium crops, as shown in this UMass bulletin, contact Peyton Ginakes.

Leek Moth: Leek moth adults emerge at 50°F, which, like ALM, is well past throughout the state and so it is also not shown in the table below. However, while it is increasingly problematic in the rest of New England (and eastern Canada), it has not been found in Maine since 2021. This pest will be covered more in a future newsletter.

GDD accumulations from Maine Climate Office. Values with an * indicate the 390 GDD threshold for onion maggot emergence is met.

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.

The University of Maine System is an equal opportunity institution committed to nondiscrimination.