Category: Disease

White Rot of Onion and Garlic

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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.

Spots on Honeycrisp Apples: What are They and How to Differentiate Them?

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Spots on Honeycrisp Apples: What are They and How to Differentiate Them?

 Zarah Ahmed, Candidate for B.S. in Physiology and Neurobiology & Macarena Farcuh, Ph.D., Assistant Professor and Extension Specialist University of Maryland, College Park

What are physiological disorders?

Physiological disorders can be defined as abnormal growth patterns that can impact the external and internal conditions of fruit. They are not caused by insects, pests, or mechanical damage, but rather by environmental factors or production practices (pre- and postharvest). The development of physiological disorders will directly impact fruit quality, thus increasing fruit losses and reducing fruit marketability and profitability.

Particularly for Honeycrisp apples, there are different types of physiological disorders that can develop both on the tree as well as during storage, but only some of them are characterized by the development of spots in the skin and sometimes flesh. Some of these include bitter pit, lenticel blotch pit and lenticel breakdown. Being able to differentiate among these will be beneficial in identifying them properly and working to prevent their future incidence.

Continue reading Spots on Honeycrisp Apples: What are They and How to Differentiate Them?

Tomato and Blossom End Rot

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By Jerry Brust, IPM Vegetable Specialist, University of Maryland

This is just a reminder with the usual summer weather we have had lately with stretches of very high temperatures and the occasional very heavy down pour we have had over the last few weeks blossom end rot can become a real problem in tomatoes. Sunny days will suck water through a plant quickly and the downpours will disrupt calcium movement through the plant. The key is to try and maintain consistent soil moisture while the fruit is developing. Easier said than done I know, but soil moisture levels need to be monitored as best as they can. When you see something like figure 1, with all the tomatoes on a cluster with blossom end rot you know the soil moisture fluctuated greatly over a fairly long period of time. Applying some foliar calcium sprays may help, but the applications can’t overcome poor soil moisture management.

Cluster of green tomatoes with blossom end rot ( browning at the base).
Fig. 1 All the tomatoes on this one cluster have blossom end rot—indicating poor soil moisture management. Photo By J. Brust. 

Unusual Disease of Garlic Scapes Found in Maryland

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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.

Blossom End Rot Common so Far this Season.

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Blossom End Rot Common so Far this Season.

By Jerry Brust, UME

This summer has been unusual as it has been about normal for temperatures if not a little cooler, but we have had greatly varying amounts of rainfall over the last month. Some areas have remained dry with storms just missing farms while others have been hit with some heavy rains. This can make watering vegetables challenging to avoid problems such as blossom end rot, which is caused by a calcium deficit in the developing fruit.

Figure 1. Fig 1. Several different vegetables with blossom end rot.

Calcium (Ca) moves to the plant via mass flow, i.e., where dissolved minerals like calcium move to the root in soil water that is flowing towards the roots. As it passes through the plant Ca is deposited in tiny amounts into the fruit. If anything slows or interrupts this stream the tiny amount of Ca needed at that moment is not deposited and the area furthest from the top of the fruit suffers—resulting in blossom end rot (BER). I have seen more BER this year on a large number of different vegetables than I have in the past several years (fig 1).

Figure two shows how precise and constant the Ca flow in a plant has to be to supply just the right amount of Ca at the right time. The large fruit on this particular plant developed before there was a Ca interruption, but the fruit a little younger suffered a Ca interruption, with the smallest (youngest) fruit suffering the greatest Ca interruption. At the time it was taken tissue analysis from this same plant showed that calcium was in the moderate range when the blossom end rot took place, demonstrating the importance of irrigation and water supply to reduce blossom end rot. Not much you can do about no rains or heavy rains, except try to maintain as even a water supply to your vegetables as is possible and remove any fruit from the plant you find that has blossom end rot.

Fig. 2 Older larger fruit received enough Ca, but younger (smaller) fruit did not so they are showing signs of blossom end rot.

 

 

 

 

 

Tomato Pith Necrosis in Maryland

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Tomato Pith Necrosis in Maryland

By Jerry Brust, UME

In the last week a few tomato fields in Maryland were found with the same disease called tomato pith necrosis. Just about all the problem tomatoes were from early planted fields. Tomato pith necrosis is caused by the soilborne bacterium Pseudomonas corrugata. Although in the past this disease occurred sporadically in Maryland, over the last few years it is appearing more frequently. Tomato pith necrosis usually is found in early planted tomatoes when night tempera­tures are cool, but the humidity is high, and often plants are growing too rapidly because of excessive nitrogen application. We have had a spring/early summer with some cool nights and high humidity.

Fig. 1. Beginning of pith necrosis- leaves anywhere on plant can turn yellow. Photo by G.Brust, UMD

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Tomato Spotted Wilt Virus in Tomatoes

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Tomato Spotted Wilt Virus in Tomatoes

By Jerry Brust, UME

A few high tunnels and even a couple of tomato fields have been found with tomato spotted wilt virus (TSWV) in Maryland. The high tunnel finds were not too surprising

but the fields were, as we usually do not see field infections until much later into the season. TSWV has also been found in greenhouse and field production of cut flowers. So it appears this virus is more common this year than it usually is, most probably due to greater thrips populations being present in our greenhouse production areas.

Fig. 1 Tomato leaves with TSWV symptoms

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Strategies for Effective Management of Botrytis and Anthracnose Fruit Rot in Strawberries

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Strategies for Effective Management of Botrytis and Anthracnose Fruit Rot in Strawberries

Dr. Mengjun Hu, Univ. of Maryland, and Kathy Demchak, Penn State Extension

Managing gray mold (Botrytis) on strawberries is increasingly challenging because of fungicide resistance development, plus a new Botrytis species that is less susceptible to fungicides is becoming common in the mid-Atlantic region. Resistance to certain fungicides is also a problem in management of anthracnose fruit rot.  This article describes disease management strategies designed to slow further resistance development, while also providing specifics for managing our two most common fruit rots.

Gray mold on strawberry. Photo: Kathy Demchak, Penn State Extension

First, what’s new with Botrytis.  There are at least 4 species of Botrytis that can infect strawberries, but only two of them have been commonly found in the region. Botrytis cinerea, the species traditionally infecting strawberries, is present nearly everywhere and affects many horticultural crops.  Recently another species, Botrytis fragariae, has also been found and as its name indicates, is more specific to strawberry plants. It appears to overwinter on strawberry plant tissue, and preferentially colonizes blossoms early in the spring, causing them to “turn brown and dry up”.  While sometimes only one of these species is present, both can be present at the same time in a field and even in the same blossom.  Using certain fungicides selects for resistant strains of either species, and also preferentially selects for one species over the other.  This means that both species have resistance to multiple fungicide groups, and both species can survive in fungicide-treated fields.

How can you tell if the newer species of Botrytis might be present in your fields?  While B. cinerea (the traditional species) is often isolated from both flowers and fruit, B. fragariae (the new one) is often isolated from flowers, and it has been shown that B. fragariae infection was much more aggressive on strawberry flowers than fruit. If you see larger-than-usual numbers of blossoms turning brown and shriveling (not to be confused with frost damage, which blackens the center of the flower), it may be prudent to choose fungicides as if B. fragariae presence had been confirmed in your field.  If you see no more symptoms on the flowers or buds than usual, you may be able to assume that the new species isn’t present, or at least not to a great extent.

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Apple Scab Alert!

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Despite cooler temperatures during the past few days, apple scab (Venturia inaequalis)  spores continue to mature. A significant apple scab infection event is forecasted April 5 to 7, 2022, in Pennsylvania and surrounding area. For those with green tissue present on their apple trees, protection is necessary for this event.
The apple scab fungus overwinters in infected leaves.  As it warms up in the spring they mature and produce spores that are discharged into air currents and carried to developing apple buds. The disease negatively affects fruit size and quality (due to blemishes and poor ripening). Over time, repeated defoliation from the disease reduces tree vigor, growth, and yield. 
Apple scrub leaf lesions.
Photo by Penn State Department of Plant Pathology & Environmental Microbiology Archives , Penn State University, Bugwood.org
Apple scab produces gray-to-olive green lesions on the leaves ( as seen above), but may also be seen on blossoms, sepals, petioles, pedicels, and fruit. On leaves, lesions first appear on the undersides of young leaves in the spring as they unfold and are exposed to infection. Leaves are susceptible to infection for about 5 days after they unfold. Fruit may become infected at any time in its development. Typical fruit lesions are distinct, almost circular, rough-surfaced, olive-green spots up to ¾ inch in diameter. 
Control information can be found on Penn State Extension website:  https://extension.psu.edu/2022-disease-update-apple-scab-infection-event-april-5-7-2022 

Are your peppers wilting? Are your melons squishy? Check for Phytophthora blight

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Written by Neith Little, Urban Agriculture Extension Agent
Reviewed by Dr. Gerald Brust, IPM Vegetable Specialist and Andrew Kness, Agriculture Extension Agent

Phytophthora blight plagues peppers and melons

The hot wet weather this summer has been the perfect conditions for Phytophthora blight of vegetables. I know of at least one urban farm where this vegetable disease has been confirmed by the University of Maryland Plant Diagnostic Lab.

Vegetables in the squash family and the tomato/pepper/eggplant family are particularly prone to infection with Phytophthora blight. This disease is particularly destructive and difficult to manage. 

What symptoms to watch for: Watch for suddenly wilting plants in wet fields or beds, with brown plant stems at the soil surface and soft, rotting fruit. 

Image above: Phytophthora symptoms on pepper plant. Note wilted leaves and brown stem near base of plant. Photo by Don Ferrin, Louisiana State University Agricultural Center, Bugwood.org
Image above: Phytophthora symptoms on pepper plant. Note wilted leaves and brown stem near base of plant. Photo by Don Ferrin, Louisiana State University Agricultural Center, Bugwood.org
Phytophthora symptoms on watermelon. Note soft “water-soaked” circular lesions on fruit, with white fungal-like growth. Photo by Jason Brock, University of Georgia, Bugwood.org
Image above: Phytophthora symptoms on watermelon. Note soft “water-soaked” circular lesions on fruit, with white fungal-like growth. Photo by Jason Brock, University of Georgia, Bugwood.org

How to manage the disease

  1. Confirm your diagnosis: If you suspect you have Phytophthora blight, seek help confirming the diagnosis. 
  2. Protect your healthy beds. Avoid tracking soil from an infected bed into a clean area. Clean harvesting and weeding tools well. 
  3. Promote good drainage. Avoid overwatering, use raised beds where possible. 
  4. Plant disease-resistant varieties. Here is a list of phytophthora-resistant pepper varieties: https://go.umd.edu/peppervarieties 
  5. Remove infected plants. Put them in the trash or bury them far from your growing area. Do not put in compost that will go back onto the farm. 
  6. Rotate crops. In a bed where you have had Phytophthora, avoid cucurbit crops and solanaceous crops for the next three years. 
  7. Chemical management can be used in conjunction with the above practices. Several fungicides are labeled for Phytophthora blight on vegetable crops. Consult the Mid-Atlantic Commercial Vegetable Recommendation Guide for a list of products and application methods. Always read and follow the label prior to any pesticide applications.

Citations and additional reading: