Gazette Gardener: Two new pests can take a toll on garlic, onions

If you’re growing ornamental alliums, garlic, onions or leeks, you’re going to want to know about tw
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If you’re growing ornamental alliums, garlic, onions or leeks, you’re going to want to know about two new troublemakers making themselves known in our region.

One is the leek moth and the other is a difficult-to-manage nematode that was positively identified in Washington County with suspected cases in Schoharie and Fulton counties.

A nematode is a microscopic roundworm. You can’t see them with the naked eye, but you can tell when they’ve been around by the damage they do.

If you think you have either problem in your garden, contact your local Cornell Cooperative Extension office for verification. In Saratoga County, CCE assistant director Sue Beebe said she recently learned about the nematodes and has already seen an example of their destructive nature after one home gardener with 1,900 plants had 80 percent of his crop infected.

Because this is the time of year when garlic growers begin to harvest, it is likely that others will become aware of this plant parasite.

Nasty nematode

The garlic stem and bulb nematode may have been lurking in our gardens unrecognized for a few years as it mimics the damage done by Fusarium basal rot, a more common fungal problem, Beebe noted.

With the confirmed case in Washington County, there will likely be more cases in our area, said Crystal Stewart, regional small fruit and vegetable specialist for CCE.

Part of the problem is the spread of the disease through garlic bulbs bought, traded and sold at garlic festivals and the distribution of garlic bulbs between backyard gardeners. At festivals, “people come from all over the state even from outside the country,” she said. With confirmed cases of the nematode in western New York, Stewart said it was only a matter of time before the nematode surfaced here.

Beebe said that since garlic is one of those crops gardeners share, beware that you may be getting more than you bargained for. “There is the potential that the nematodes will be in the saved bulbs that we keep to pass along year to year. The bulbs may look healthy but they may not be healthy,” she advised.

The nematode feeds on plants by puncturing and sucking the cell contents through a needle-like mouth part called a stylet. It lives within the plant, feeding in stems, leaves and bulbs. The outward appearance of the garlic may be stunted and the leaves may be thickened or distorted, Stewart said. But the real damage is inside the plant, under the soil.

As you harvest your garlic in the next few weeks, be observant. If you notice that half the roots are missing or that the bulb is underdeveloped, this is an indication that the nematode is present.

“Garlic should have a strong root system. If you notice roots missing or the bottom of the garlic starting to crack, these are clear signs of nematodes,” Stewart said.

Other symptoms are a light brown discoloration of the cloves, or cloves that are soft, mealy or spongy in texture and distorted. At later stages of infection, the cloves separate from the basal plate, become cracked and develop a soft rot.

If the roots are half gone, you can still eat the garlic, but don’t eat them if they are discolored, Beebe warned.

What should a gardener do if they suspect they have an issue? Bring a few bulbs to your local CCE. Don’t assume it is nematodes, it may not be. “It could be fusarium or several other possible issues,” Beebe said. Having the bulbs looked at won’t cost you anything, she said.

Controls for the nematode have not been determined. Beebe indicated that nematodes are difficult to manage. However, it is known that they stay in one area, so crop rotation is an effective option, she said.

Leek moth

The leek moth is a pest of members of the allium family including onion, garlic, leeks, chives and shallots. It can also feed on ornamental alliums.

It has been a problem in Canada for more than a decade, and last year crossed the border into Plattsburgh. Now there are confirmed cases in Clinton and St. Lawrence counties, and unconfirmed cases in Franklin and Jefferson counties, according to Dr. Richard Hoebeke, entomologist with Cornell University who has been verifying cases.

“I suspect it is widely distributed across the northern counties. The species has slowly been spreading for the last couple of years. I have heard it is in Vermont as well,” he added.

Commercial growers aren’t finding the moth on their crops as much as small growers and home gardeners. Hoebeke said this is likely because the large commercial growers follow an insecticide schedule for other pests and the moth is being controlled.

“Ma and Pop growers of garlic and onions are probably not treating their crops. It is from these small growers that we are learning about the spread,” he said.

Although the damage can be extensive, the moth itself is small — only about three-eighths of an inch long. It’s speckled brown, white and black, with a distinctive white spot halfway down its outer pair of wings.

The moths are nocturnal. In a fact sheet created by Amy Ivy at the Clinton County CCE office, the life cycle and damage were explained. The larvae are creamy yellow, slender and less than a half inch long when fully grown. The larvae morphs into a pupa that has a unique netlike structure over the cocoon and is attached to dying foliage.

Though you may not spot the moth, you will see the damage the larvae cause. On onions and chives, the larvae feed on the tissue inside hollow leaves and create a ‘window pane’ look where they have eaten. If you suspect you have an issue, split open the leaves and look for the larvae.

On garlic and leeks, the larvae feed on the leaf surface but usually do not feed all the way through the leaf. On these crops you can find the larvae hiding in the folds of leaves.

Whitish dead patches or streaks running lengthwise down the leaves, and damage on the flowering scapes of hardneck garlic are examples of the effects of their feeding.

The leek moth isn’t the only pest on these crops. Thrips can damage onions and leeks, as can slugs.

How can you tell what’s eating your plants? Ivy wrote that thrips feed on the outside and cause damage that look like speckling. Slugs usually eat right through a leaf, leaving it looking shredded.

As of today, there are no pesticides specifically labeled to control the leek moth in New York.

But there are cultural practices that will help the home gardener.

The adults overwinter in plant debris, so keep the garden clean. Inspect your plants and crush and dispose of any pupa you find. Next season, rotate the crop and place a row cover over the plants to create a barrier and prevent adults from laying eggs.

It’s hard to write “Happy Gardening” after a column like this. But by being observant and keeping on top of the pests that are around, we can make our gardens happy and that makes us, the gardeners, happy. Right?

Natalie Walsh is a horticulturist, speaker and garden consultant. She can be reached at [email protected]

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