Allium Leafminer Moving South in Maryland

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

The new pest of onion, leek and garlic, the Allium leafminer, is moving south in Maryland. It was first observed in Maryland in Cecil Co. in 2017, but now the fly’s tell-tale marks (Figs. 1 and 2) have been found in a Baltimore City chives planting. This new pest was first found in Lancaster County, Pennsylvania in December 2015. Unfortunately, it is my guess that the pest is now probably in many northern/central areas of Maryland. New transplants or seedings of onions or leeks should be watched closely for the tell-tale signs of the fly’s damage which are several very small white dots in a row along the leaf of an allium plant (Figs. 1 and 2).

Figures 1 and 2. Tell-tale marks on allium leaf made by Allium leafminer females

Penn State has a great deal of good information about the new pest which can be found at: Penn State Allium Leafminer Pest Alert page. Growers should look for these tell-tale signs on any newly planted allium species, but especially on leeks. You can cover any Allium planting with row cover to keep the flies off or if needed treat with insecticides as found in the 2019 Mid-Atlantic Commercial Vegetable Recommendations guide.

Virus Problems Found in Garlic Early This Year

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

Last year I had an article sometime in July about what I called “garlic viruses” which I had not seen in our area before, but I know must have been around before this. This year some garlic growers are noticing this virus complex already in their fields and I am not sure if that is because they are more aware of it or because the virus complex is expressing itself earlier. Symptoms of virus infection are plants that display yellowing tips on many leaves with some that are completely yellow (Fig. 1). If you look closely at the yellow leaves you’ll see mottling or striping on the leaves (Fig. 2). Symptoms are usually more pronounced on young leaves. Infected plants are stunted and bulb size can be reduced. Garlic crops infected with certain of these viruses are more susceptible to weather conditions like extreme heat, and do not keep well post-harvest.

What I am calling garlic virus is caused by several different viruses that can be grouped under the name “Potyvirus”; all symptomatic garlic that was tested this year was positive for Potyvirus. Some people lump these viruses under the name “garlic mosaic”. In this case garlic mosaic is thought of as a disease caused by one or more viruses belonging to the Potyvirus group which includes onion yellow dwarf virus, leek yellow stripe virus, and others. These viruses can be transmitted through the planting stock or by aphids and it is thought because garlic is clonally propagated probably most of the planting stock is infected with some type of virus. These viruses are usually mild and do not seriously affect yield. The problem comes in when the plants are infected with several different Potyviruses, and then there can be moderate to severe yield reductions. We may have had more aphid movement earlier in the year because of the mild winter and early spring, which may have increased additional virus infections in garlic plantings. You cannot reduce virus transmission by spraying pesticides. Any garlic with symptoms should be watched and possibly harvested early or rouged out if yellowing and decline increase in the coming weeks.

Figure 1. Garlic plants showing symptoms of infection with virus complex

Figure 2. Streaking, striping on leaves of garlic infected with virus complex

New Pest, Allium Leafminer, Present in Neighboring States

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

The allium leafminer (ALM) or onion leafminer is an invasive species that was detected in Lancaster, PA in 2015. This was the first confirmed detection in the Western Hemisphere. Since this detection, it has spread to New Jersey, New York, and Maryland. It has not been detected in Delaware but is something you should be aware of.

The allium leafminer is known to infest species in the genus Allium. This includes leeks, onions, garlic, chive, shallot, and green onion. The adults, small grey or black flies with a yellow or orange marking on the top and front of head, emerge in late winter through spring (March-May) and begin laying eggs at the base of plant stems. The larvae mine leaves, moving downward to the base of leaves or into bulbs where they pupate. Pupae may move into soil. During the summer months, ALM undergoes diapause in the pupal stage before developing into adults which emerge in the fall (September/October). This generation also attacks Allium spp. before overwintering as pupae.

Here are a couple links from Penn State University and University of Maryland for more information on allium leafminer including pictures of adults, pupae, monitoring, damage, and management:

http://ento.psu.edu/extension/vegetables/pest-alert-allium-leafminer

https://extension.umd.edu/learn/allium-onion-leafminer

If you suspect you have damage or a life stage of the allium leafminer, please contact Steve Hauss at the Delaware Department of Agriculture (302-698-4500 or Stephen.hauss@state.de.us) for official confirmation since this pest has not been detected in Delaware.

Garlic Viruses

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

In the past I have talked about several different problems with garlic, be it mites, nematodes, fungal or bacterial diseases. However, this year I am seeing something different that I will just call ‘garlic viruses’ that may have been there in past years, but is more prominent now for some reason. Symptoms of virus infection are plants that display yellowing tips on many leaves with some that are completely yellow (Fig. 1). If you look closely at the yellow leaves you’ll see mottling or striping on the leaves (Fig. 2). Symptoms are usually more pronounced in young leaves. Infected plants are stunted and bulb size can be reduced. Garlic crops infected with some of these viruses are more susceptible to weather conditions like frost, and do not keep well in storage.

What I am calling garlic virus is caused by several different viruses that can be grouped under the term “potyvirus”–the garlic we tested was positive for potyvirus. Some people lump these viruses under the name “garlic mosaic”. In this case garlic mosaic can be thought of as a disease caused by one or more viruses belonging to the potyvirus group, which includes onion yellow dwarf virus, leek yellow stripe virus, and others. These viruses can be transmitted through the planting stock or by aphids and it is thought by some that because garlic is clonally propagated that much of the planting stock is infected with some type of virus. These viruses are usually mild and do not affect yield to any great extent. The problem comes when the plants are infected with several different potyviruses, and then there can be moderate to severe yield reductions. For instance, when infected with 2 or more potyviruses yield loss in garlic bulbs can be as much as 70% or germination rates can be reduced by 55%. Some hosts of these potyviruses include: garlic, great-headed garlic, leek, pearl onion, wild garlic, onion and shallot. The two most common aphid vectors are the green peach aphid (Myzus persicae) and the black bean aphid (Aphis fabae).

Management Usually virus infections are not a problem, but for this season they seem to be more prominent. We will need to see if this is an aberration or a trend. If the frequency of the virus continues then growers will need to move to virus-free stocks that are produced from meristematic tip culture. The use of this “virus-free” stock should result in significantly greater yields.

garlicvirus1

Figure 1. Garlic plants showing symptoms of garlic virus infection

garlicvirus2

Figure 2. Streaking and striping on leaves of garlic infected with virus

Garlic Problems — Again

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

Last year at about this same time there were calls from growers about their garlic plantings turning yellow and wilting (Fig. 1). When dug up the bulbs were often times blackened and rotting with some or much of the basal plate or roots missing (Fig. 2). The calls are coming in again this year with the same complaint and unfortunately, the same problems—bulb mites and garlic bloat nematode.

allium1

Figure 1. Bulb mite/bloat nematode infested garlic field

Bulb mites are a problem of garlic and sometimes of onion that usually go unrecognized. These pests can reduce stands, decrease plant vigor, and increase post-harvest diseases by their feeding on the roots and the stem plate. Bulb mites have a very wide host range, but cause most of their damage to onions and garlic. These mite pests are usually not seen on the bulb and prefer crawling into crevices between the roots and stem plate. Early in the growing season, bulb mites can cause poor plant stands and stunted growth as they feed on roots. Infested plants easily can be pulled out of the soil because of the poor root growth (Fig. 2).

The mite is bulb shaped with its legs moved forward and a bulbous rear end and many long fine hairs (Fig. 3). The mouthparts and legs are purplish-brown while the main body is creamy white. These mites have been described as looking like tiny pearls with legs. The mites are extremely small (from 0.02 to 0.04 inches) and are very slow moving. They are usually found in clusters underneath scales and at the base of the roots.

The garlic bloat nematode Ditylenchus dipsaci can destroy a crop of garlic in one season. Symptoms of bloat nematode in garlic plants include: bloated, twisted, swollen leaves, distorted and cracked bulbs with dark rings (Fig.4). These nematodes also can move to the inflorescence and remain in seeds for long periods of time in some plant species, i.e., beans, clover, and alfalfa where they are major sources of nematode dispersal. The nematodes can be spread around fields by equipment or on clothing and shoes. Garlic bloat nematodes can overwinter in soil or crop debris.

It is not just the direct feeding of the nematodes and mites on garlic and onions that causes problems, their feeding also allows pathogens to enter through the wounds they create. These wounds are very good entry points for pathogens like Fusarium spp., Sclerotium cepivorum (causes the disease white rot), and various soft-rotting bacteria. The white rot fungus does best in cool temperatures, and symptoms include white fungal growth on the stem or bulb with small, dark structures called sclerotia in the decayed tissue. Later in the season, higher than normal amounts of soft rot and Fusarium dry rot may be seen because of the wounds caused by these mites (as we saw in a couple of the garlic fields).

There is no program that certifies garlic as nematode-free. Commercial suppliers of garlic bulbs are aware of this important problem, and may send a portion of their crop to a laboratory for nematode testing, but this does not certify a crop as nematode-free. Because the nematode and mite can survive for long periods on infected plant material, to prevent build-up of the nematode or mite populations in a field, you MUST rotate away from any Allium crops (garlic, onions, and leeks) and control nightshades for at least 4 years. DO NOT keep any bulbs or seed from an affected field no matter how clean it looks. You should start from fresh seed or bulbs. Rotation to areas of the farm that have not had garlic or onion plantings for many years with new garlic or onion seed is the best method of control. Growers can, however, use soil fumigants to reduce or eliminate the nematodes from infested areas of the field. Growers also can use bio-fumigant cover crops that can be planted after harvesting garlic. Mustard and sorghum-sudangrass have been shown to reduce nematode populations due to the bio-fumigant constituents they produce. Be sure to clean equipment and storage areas with meticulous sanitation techniques.

allium2a allium2b Figure. 2 Infested garlic bulbs, misshapen or rotting bulbs, sometimes roots are intact other times there are no roots

 allium3Figure 3. Bulb mite

allium4Figure 4. Severe garlic bloat nematode damage to the two bulbs on the right vs. non-infested bulbs on the left

Garlic Bloat Nematode Found in Several Garlic Samples

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu and Karen Rane, Extension Specialist Entomology, University of Maryland rane@umd.edu

This must be a bad year for garlic because besides finding bulb mites we also have found garlic bloat nematode in several samples of damaged garlic. I wrote about this nematode last year and advised garlic growers to watch for it and to test any bad looking bulbs for it. This year growers are sending in their bad looking garlic bulbs and unfortunately many are infested with this nematode. The garlic bloat nematode Ditylenchus dipsaci can destroy a crop of garlic in one season. It probably came from Canada in garlic that was imported for food, but was planted as seed garlic. The problem then spread through distributors because there is no certification program for seed garlic and is it now widespread throughout New York. Symptoms of bloat nematode in garlic plants include: bloated, twisted, swollen leaves, distorted and cracked bulbs with dark rings (Fig.1).

Infested tissues become spongy, distorted and predisposes the plant to other problems like fusarium or white rot (Fig. 2) and bulb mites. These nematodes also can move to the inflorescence and remain in seeds for long periods of time in some plant species, i.e., beans, clover, and alfalfa where they are major sources of nematode dispersal. The nematodes can be spread around fields by equipment or on clothing and shoes. Garlic bloat nematodes can overwinter in soil or crop debris. If a grower has purchased or brought in new planting material over the last few years, especially if it came from Ontario or New York, you may have this pest. If you have not made any new introductions in a while you are probably safe. If you have garlic bulbs that look something like they do in figure 1 or 2 you should send a sample to a nematode laboratory for testing.

 

Fig. 1 The lack of roots on one side of plate and bulb deformation can be indicators of bloat nematode infection.

To prevent build-up of the nematode populations in a field, rotate away from any Allium crops (garlic, onions, and leeks) and control nightshades for at least 4 years. Another method to reduce levels of bloat nematodes in the soil is to keep the fields where garlic was grown moist, because bloat nematodes cannot survive for long periods in moist soils. They can persist for several years though, in dry soil and on dry plant residue. Bloat nematodes can actually survive better in dried crop debris than in soil.

Growers can use soil fumigants to reduce or eliminate the nematodes from infested areas of the field. Growers can also use bio-fumigant cover crops that can be planted after harvesting garlic. Mustard, sorghum-sudangrass have been shown to reduce nematode populations due to the bio-fumigant constituents they produce. Be sure to clean equipment and storage areas with meticulous sanitation techniques.

Fig. 2 Non-infested garlic bulbs (left) and infested garlic bulbs (right) with bloat nematode

Bulb Mite Found in Problem Garlic Fields

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu and Karen Rane, Extension Specialist Entomology, University of Maryland rane@umd.edu

Several garlic fields have been having problems the last few years with rotting bulbs. Some seedcorn maggot was found feeding on the bulbs and we thought this was the major problem. But damage continued beyond what the maggots could do. On a second look very tiny bulb mites were found in the damaged garlic. Bulb mites are a problem of garlic and sometimes of onion that usually goes unrecognized (as in this case). These pests can reduce stands, slow plant vigor, and increase post-harvest diseases by their feeding on the roots and the stem plate. Bulb mites have a very wide host range, but cause most of their damage to onions and garlic. These mite pests are usually not seen on the bulb and prefer crawling into crevices between the roots and stem plate.

The best way to determine whether these mites are present is to carefully dissect the region where the roots and bulb come together. The mites also could be under one or two layers of scales at the lower end of the bulb. There are usually other mites present, but with a hand lens the bulb mites usually can be identified from other mites.

The mite is bulb shaped with its legs moved forward and a bulbous rear end and many long fine hairs (Fig. 1). The mouthparts and legs are purplish-brown while the main body is creamy white. These mites have been described as looking like tiny pearls with legs. The mites are extremely small (from 0.02 to 0.04 inches) and are very slow moving. They are usually found in clusters underneath scales and at the base of the roots.

It is not just the direct feeding of these mites on garlic and onions that causes problems, but also their feeding allows pathogens to enter through the wounds they create. These wounds are very good entry points for pathogens like Fusarium spp., Sclerotium cepivorum (causes the disease white rot), and various soft-rotting bacteria. The white rot fungus does best in cool temperatures, and symptoms include white fungal growth on the stem or bulb with small, dark structures called sclerotia in the decayed tissue (Fig. 2). Early in the growing season, bulb mites can cause poor plant stands and stunted growth as they feed on the plants (Fig. 3). Infested plants easily can be pulled out of the soil because of the poor root growth. Later in the season, higher than normal amounts of soft rot and Fusarium dry rot may be seen because of the wounds caused by these mites (as we saw in a couple of the garlic fields).

Figure 1. Garlic bulb mites greatly magnified

Bulb mites survive in the soil on organic matter left behind from the previous crop. As long as there is decaying allium vegetable matter in the soil, bulb mites can survive in the field. The best way to control bulb mites is to allow the vegetation from the previous crop to breakdown before any new crop, especially garlic or onions are planted again. Low areas of the field that stay wet and have high levels of organic matter are especially prone to greater bulb mite survival. These mites may also come into a clean field on infested garlic cloves. The use of clean garlic clove seed or seed that has been hot water treated will control these pests. Effective hot water treatment will also, unfortunately, reduce bulb vigor.

 Figure 2. White rot (white fungus) with microsclerotia (arrows) in garlic stem

 Figure 3. Garlic plants damaged by bulb mite feeding and invasive fungi

Nematode Pest Recently Found in New York Garlic Fields May Also Affect Mid-Atlantic Growers

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

There is a new ‘old’ pest infecting garlic and onions in New York and other New England states that has been found as far south as Pennsylvania. It is the garlic bloat nematode. The garlic bloat nematode Ditylenchus dipsaci is capable of severely damaging a field of garlic very quickly. It probably came from Canada in garlic that was imported for food, but was planted as seed garlic. The problem then spread through distributors because there is no certification program for seed garlic and it is now widespread throughout New York. Symptoms of bloat nematode in garlic plants include: bloated, twisted, swollen leaves, distorted and cracked bulbs with dark rings (fig.1). Infested tissues become spongy, distorted and predisposes the plant to other problems like fusarium or white rot (fig. 2). Garlic bloat nematodes can overwinter in soil or crop debris and can move to the inflorescence and remain in seeds for long periods of time in some plant species, i.e., beans, clover, and alfalfa, which act as major sources of nematode dispersal. The nematodes can be spread around fields by equipment or on clothing and shoes. If a grower has purchased or brought in new planting material over the last few years, especially if it came from Ontario or New York, you may have this pest. If you have not made any new introductions in a while you are probably safe. If you have garlic bulbs that look something like figure 1 or 2 you should send a sample to a nematode laboratory for testing.

To prevent build-up of the nematode populations in a field, rotate away from any Allium crops (garlic, onions, and leeks) and control nightshades for at least 4 years. Another method to reduce levels of bloat nematodes in the soil is to keep the fields where garlic was grown moist, because bloat nematodes cannot survive for long periods in moist soils. They can persist for several years though, in dry soil and on dry plant residue. Bloat nematodes can actually survive better in dried crop debris than in soil.

Growers can use soil fumigants to reduce or eliminate the nematodes from infested areas of the field. Growers can also use bio-fumigant cover crops that can be planted after harvesting garlic. Mustard, sorghum-sudangrass have been shown to reduce nematode populations due to the bio-fumigant constituents they produce. Be sure to clean equipment and storage areas with meticulous sanitation techniques.

Figure 1. The lack of roots on one side of plate and bulb deformation can be indicators of bloat nematode infection.

Figure 2. Non-infested garlic bulbs (left) and infested garlic bulbs (right) with bloat nematode