Category: Allium

White Rot of Onion and Garlic

Emily Zobel on

White Rot of Onion and Garlic

By Jerry Brust, UME and Karen Rane, UMD Plant Diagnostic Lab

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)). White rot is NOT the same as white mold, which is caused by Sclerotinia sclerotiorum, which has a very large host range (tomatoes, peppers and 170 other plant species); white rot only infects Allium species.

White rot on base of a garlic plant.
Fig. 1.) White rot on garlic plant. Photo By K. Rane, UMD,

Leaves of Allium plants with white rot exhibit yellowing, dieback, and wilting. Under ideal weather conditions, white mycelial growth can develop on the bulb. As the disease progresses, the mycelium becomes more compacted with numerous small, spherical black bodies (sclerotia) forming on this white mat (fig. 2). These sclerotia are the overwintering structures of the pathogen and are approximately the size of a pin head. As the disease progresses, these sclerotia are eventually released into the soil. Infected plant roots will rot, making the plant easily pulled from the soil. Disease development is favored by cool, moist soil conditions. The soil temperature range for infection is 50°-75°F, with an optimum of 60°- 65°F. At soil temperatures above 78°F, the disease is greatly inhibited. Soil moisture conditions that are favorable for onion and garlic growth are also best for white rot development.

Spherical black bodies a of mycelial growth of the white rot fungus on garlic.
Fig. 2) Sclerotia (Spherical black bodies a of mycelial growth ) of white rot fungus on garlic. Photo By G. Brust, UMD.

An increase of white rot in a field that has had several Allium crops may go unnoticed for a time as sclerotia numbers increase and disperse. One sclerotium per 20 pounds of soil will cause disease and results in measurable crop loss. The sclerotia will lay dormant until root exudates, exclusively from an Allium species, stimulate germination. Root exudates from non-Allium species will not stimulate the germination of white rot sclerotia. Cool weather is needed for both sclerotia germination and mycelia growth. Mycelia will grow through the soil until they encounter an Allium root at which time the fungus initiates infection. Mycelia can grow from one plant to a nearby plant, allowing the pathogen to move between plants.

Management of white rot should focus on disease avoidance by not introducing the pathogen into a field. Sclerotia can spread throughout a field, or from field to field, through the movement of soil, equipment, or plant material (especially garlic cloves). Sanitation is important to prevent sclerotia from moving from an infested field to a clean field. Plant only clean stock from known origins that has no history of white rot. Always clean soil off of equipment and sanitize with quaternary ammonia before moving to another field. The Allium crops from an infested field should not be used as seed. Rotation alone will not control white rot because sclerotia can survive in the soil for 20-40 years. If the disease is found, reducing or eliminating irrigation will reduce the damage to the current crop but will not stop the spread of the disease.

Because the fungus is vulnerable to temperatures above 115°F, dipping seed garlic in hot water is a possible preventive measure that will reduce the amount of pathogen but will not completely eliminate it. Temperature control is important when using this method because temperatures above 120°F may kill the garlic. There are other cultural and organic practices (i.e., biofumigation and solarization) that a grower might try to fight this disease and these can be found at: https://rvpadmin.cce.cornell.edu/uploads/doc_479.pdf

Chemical applications can be made for white rot management and include for onion tebuconazole applied in a 4-6 inch band over or into the furrow at planting or via chemigation. For garlic an in-furrow at-planting application using iprodione or tebuconazole or fludioxonil can reduce disease incidence, however there are crop rotation restrictions with the use of these chemicals so be sure to check the Mid-Atlantic Commercial Production Recommendations guide for more details.

One other note is that the presence of bulb mites can exacerbate disease problems by opening the bulb up to infection from white rot and growers also will need to manage these mites.

Unusual Disease of Garlic Scapes Found in Maryland

Emily Zobel on

Unusual Disease of Garlic Scapes Found in Maryland

By Jerry Brust, UME and Karen Rane, UMD Plant Diagnostic Lab

A grower noticed over the last few years lesions developing on their garlic scapes which then collapsed in the field. In previous years these collapsed scapes amounted to only a small number, but this year the losses are much greater approaching 30%. Symptoms consist of sunken lesions about ¼ to ½ inch long, that cause twisting, girdling and collapse of the scape. Lesions initially are cream to tan-colored but under rainy or very humid conditions, spore production by the fungus causes lesions to turn orange (fig 1). This disease is anthracnose of garlic, a new disease to Maryland and is caused by the fungus Colletotrichum fioriniae. The fungus may survive on crop residue in the soil from a previous garlic crop or the disease may be spread by infected bulbils used for propagation. Disease development is favored by rainy or very humid weather and warm temperatures (78-88o F). Anthracnose of garlic does not affect bulbs, but scape yield could be reduced as will bulbil production.

Fig. 1 Collapsed base of scape with white lesion and orange spores (left) and twisted orange scape stem (right). Photo by M.Mclearen.

Reports from New England indicate that onion is most likely not affected by this fungus. C. fioriniae has also been reported as causing bitter rot on pear and anthracnose on celery and cherry tomato. Crop rotation away from any member of the onion family may help reduce disease incidence. Besides crops, weeds such as common lambsquarters, redroot pigweed, yellow nutsedge and common groundsel may also be infected with the pathogen but be symptomless. Because this is such a new disease of garlic, fungicide recommendations have yet to be determined. However, products that are labeled and effective against purple blight of onion may be useful against this disease.

Allium leaf miner

Emily Zobel on
Adult allium leaf miners.

Allium leaf miner Phytomyza gymnostoma is a pest on chives, scallions, garlic, onions, and leeks. Overwinter ALM across the Mid-Atlantic will be emerging soon. Start scouting for ovipositor markings made by female ALM over the next few weeks on food and ornamental Allium crops. These markers will be neat rows of white spots descending from the upper tips of allium leaves. These marks will be typically be seen on the tallest leaves first. ALM Larvae mine leaves and moves towards and into bulbs and leaf sheathes. The leaf punctures and mines serve as entry routes for bacterial and fungal pathogens.

Spring crops are usually not as hard-hit as fall crops, especially when looking at leeks, but this pest has steadily increased its geographical range each year and its damage potential. If you had some infestation last year, you will especially want to look for this pest’s signs.

Yellow sticky cards or yellow plastic bowls containing soapy water can be used for monitoring but are not affected control independently. 

 

Onion leaf blades showing round white dots made by female Allium leaf miners. Photo by Jerry Brust

 

Cultural Control: Covering plants in February, prior to the emergence of adults, and keeping plants covered during spring emergence, can be used to exclude the pest. Avoiding the adult oviposition period by delaying planting (after mid-May we think) has also been suggested to reduce infestation rates. Covering fall plantings during the 2nd generation flight can be effective. Growing leeks as far as possible from chives has been suggested. Organic

 

Chemical Control: Azadirachtin (Aza-Direct or other formulations) or spinosad (Entrust or other formulations) follow label instructions for leaf miner.

Synthetic Chemical Control: Systemic and contact insecticides can be effective. EPA registrations vary, however, among Allium crops. Check labels to ensure the crop is listed, and for rates and days-to-harvest intervals. Options that may be effective include cyromazine (Triguard), dinotefuran (Scorpion), spinetoram (Radiant), lambda-cyhalothrin (Warrior II or other formulations), and abamectin (Agri-Mek or other formulations).