Onions, Green Onions, Garlic and Leeks

Vegetable and Fruit News, Volume 13, Issue 4 (July 2022). Topics in this issue are: Disease of Garlic Scapes, Blossom End Rot, Preventing Bruising in Apples and Peaches, Corn Disease Identification, Tomato Spotted Wilt Virus, Tomato Pith Necrosis, July IPM Tips, Mowing: IWM Tool, and Upcoming Events

Fruit and Vegetable News, Volume 13, Issue 1 (April, 2022). Topic in this issue are: Check for Allium leaf miner in onions and leeks over the next few weeks, Edema problem in high tunnel tomatoes, Using Plant Growth Regulators to Improve Apple Return Bloom, Botryis Fruit Rot (Gray Mold) and Crown Rot in Strawberries, Spring Pest Scouting in Strawberries, and Early Spring Vegetable Insect Scouting Tips.

Vegetable and Fruit News, Volume 14, Issue 3 (May 2023).

If you grow leeks or onions or other Allium species, you should already be checking for the tell-tale marks left by Allium leaf miner. Allium leaf miner Phytomyza gymnostoma tell-tale marks consist of many linear small white dots (made by the female’s ovipositor) that appear in the middle towards the end of leaf blades (fig. 1) of their preferred hosts of leeks, onions, garlic and other Allium species. Spring crops are usually not as hard hit as fall crops especially when looking at leeks, but this pest has been steadily increasing its geographical range each year and its damage potential. If you had some infestation last year you will especially want to be looking for the signs of this pest.

Bulb mites (usually Rhizoglyphus spp) are a problem of garlic that easily go unrecognized. Usually growers notice a general yellowing of their garlic plants with the tips of leaves often turning brown (fig 1). If you examine the bulb it can have feeding marks on the outside of the bulb (fig. 2) or the basal plate can separate easily from the bulb (fig. 3).

If you grow leeks or onions or other Allium species, now and for the next few weeks is the time to watch for the tell-tale marks left by Allium leaf miner. Allium leaf miner Phytomyza gymnostoma tell-tale marks consist of several small round white dots (made by the female’s ovipositor) in a row that appear on the middle towards the end of leaf blades (fig. 1) of their preferred hosts of leeks, onions, garlic and other Allium species.

The weather we had in May was great for seed maggots and for some soil diseases. One very serious soil disease that affects Allium species, especially onion and garlic, is white rot, caused by the fungus Stromatinia cepivorum (syn. Sclerotium cepivorum (fig. 1)), which was found in the last few weeks in Maryland.

Southern blight is a fungal disease that is most common in the tropics and subtropics, but also is found in the SE United States in the summer. In Maryland we usually find it in the southern part of the state in counties like St. Mary’s, Charles and Wicomico. It has been found this year in more northern counties such as Frederick, Montgomery and Baltimore.

Last week I had a call from a vegetable grower who thought they might have Allium leaf miner (LM) pretty bad in their onion field. While this grower was organic (synthetic pesticide use reduces Allium LM risk) and in the northern part of Maryland I was skeptical about Allium LM being a really bad problem in onions. As I approached the onion field it did look like the pictures I have seen of Allium LM damage in Pennsylvania. The onions were broken and falling over one another and there were white marks on the onion stems (fig. 1), just like what has been seen in PA (fig. 2).

Garlic growers may be noticing symptoms of virus infection in their plants that show yellowing tips on many leaves with some that are completely yellow (fig 1). If you look closely at the yellow leaves you will see mottling or striping on the leaves (fig 2). These symptoms are usually more pronounced on young leaves. Infected plants are stunted and bulb size can be reduced. Garlic crops infected with viruses are usually more susceptible to weather conditions like extreme heat, and do not keep well post-harvest.