First 2017 Report of Cucumber Downy Mildew in Ohio

Downy mildew was confirmed this afternoon in cucumbers in West Salem (Wayne County), OH.  This follows a report of downy mildew on pickling cucumbers and cantaloupe in Essex County, Ontario yesterday, and is about 8 days earlier than our first Ohio report in 2016. Cucumber and melon growers throughout Ohio should intensify scouting, and these crops should be protected with effective fungicides. Cooler, wetter conditions the last week have been very favorable for downy mildew. In northern Oho counties, the downy mildew risk is high, so the more effective downy mildew fungicides Ranman 400SC and/or Orondis Opti A & B (co-pack) should be the core of the fungicide program.  Zing!, Gavel, Zampro or other fungicides (see table below) can be rotated into the program. A chlorothalanil (Bravo, Echo, Equus, Initiate) or mancozeb (Dithane, Manzate, Penncozeb) product should be tank mixed with the downy mildew fungicide.  Always rotate fungicides with different modes of action and follow label instructions. Remember that Orondis Opti A & B applications are restricted to 1/3 of the total fungicide applications.  We recommend applying the first Orondis application when the risk of downy mildew is highest. Under highly conducive environmental conditions, apply fungicides on a 5-7 day schedule.  When the risk is lower due to hot, dry, sunny weather, or downy mildew has not been reported in the area, the schedule may be stretched to 7-10 days. Cucumber and cantaloupe downy mildew risk is much higher in northern than in central and southern Ohio at this time.

Thanks to Chris Smedley and the Wayne County IPM scouting program for bringing this sample to us for confirmation.

Watch for Late Blight in Tomatoes and Potatoes

Our colleagues in Ontario reported late blight today on tomatoes in Chatham-Kent.  That means that inoculum of the late blight pathogen is active in the region, and given the rainy/overcast, cool conditions of the last week or so, it is time to make sure tomato and potato crops are protected from this disease.

It is very important that both potatoes and tomatoes be scouted regularly (at least twice per week) for late blight.  If late blight is suspected, it can be confirmed by bringing or sending a sample to the OSU Vegetable Pathology Lab in Wooster  or the OSU C. Wayne Ellett Plant and Pest Diagnostic Clinic in Reynoldsburg.   We appreciate hearing from you if late blight is suspected so that we can confirm and alert others.

Growers need to maintain an effective fungicide program on tomatoes and potatoes.  This should continue as long as rainy conditions, high humidity and/or heavy dews are expected.  If late blight has not been observed and weather conditions are generally dry and warm, use a protectant fungicide on a 7-10 day schedule, depending on how fast the plants are growing.  Good protectants are chlorothalanil (Bravo, Equus, Echo), mancozeb (Penncozeb, Manzate, Dithane), and to a lesser extent, copper-based products – use according to label instructions.  Under cool wet conditions when late blight is likely, or if late blight has been found on the farm or nearby, use one of the following, tank mixed with one of the above protectants: Curzate (3 day PHI),Orondis Ultra (1 day PHI),Presidio (2 day PHI), Previcur Flex (5 day PHI), Ranman (0 day PHI), or Tanos (3 day PHI). Orondis Opti (3 day PHI),  Zing! (5 day PHI) and Gavel (5 day PHI)  are pre-mixes containing mancozeb or chlorothalanil.  Note the Pre-Harvest Interval (PHI) for these fungicides ranges from 0 – 5 days.  So far, Ohio late blight strains have been sensitive to Ridomil, so that is another option (but may be risky).

For organic producers, there is a good webinar on late blight and its management in organic potatoes and tomatoes originally presented in January 2014 and featuring leading researchers on late blight from New York, Pennsylvania, Wisconsin and Florida. They mention Actinovate AG, EF-400 and Zonix as products approved for organic that can be effective in reducing late blight – but will not completely control the disease. OMRI-approved copper fungicides (Champ  WG, Nordox 75 WG, and others) generally rank highest for control among organic approved products. Actinovate, EF-400 and Zonix might be used in a program with copper. NOTE: It is critical to apply  fungicides prior to infection – these fungicides (and most for conventional use as well) are not effective once plants have become infected.

If the disease is mainly found in one or a few foci, it is a good idea to remove and destroy the diseased plants.  This is particularly important for organic tomato or potato growers.  Home gardeners should consider spraying tomatoes and potatoes with a fungicide containing chlorothalanil, and should prune out diseased tissue.  Pack up diseased plants in a plastic garbage bag and discard – don’t leave them in the field or garden or on a compost pile, where the pathogen can be released into the air and spread to other plants.



This Week in the OSU Vegetable Disease Diagnostics Lab

It has been a busy week in the diagnostics lab – we continue to receive high tunnel and greenhouse tomatoes, but open-field vegetables and herbs are now making an appearance. These are some of the plant disease problems from around Ohio received this week.

Basil downy mildew – sporulation of pathogen Peronospora belbahrii on stem

Basil downy mildew. The sample from Ashland County had unusual sporulation on the stems (see photo). We normally find lesions on leaves with spores on the underside. This might indicate a systemic infection, possibly from seeds. See my post on May 4, 2017 for recommendations on management of this disease. Note: The basil downy mildew pathogen does not cause disease in cucurbits. We have not seen cucurbit downy mildew in Ohio or in the Great Lakes region yet this year.

Early symptoms of bacterial wilt on cucumber leaves – photo by F. Rotondo

Cucumber bacterial wilt. This disease was found in Highland County. Bacterial wilt can be a very serious problem in cucumbers, melon, squash and pumpkins, among other cucurbits. It is caused by a bacterial plant pathogen – Erwinia tracheiphila – that overwinters in striped and spotted cucumber beetles. When the beetles emerge from the soil they feed on cucurbit plants and transmit the bacteria to wounded tissue. We expect that beetle survival was higher than usual this year as a result of the mild winter of 2016/2017. The beetles can damage or kill young plants even in the absence of Erwinia, but if the bacteria enter the plant it is likely to die or at least perform poorly. The earlier plants are infected, the more likely they are to die quickly. Conventional growers can control bacterial wilt with appropriate insecticides – see the Midwest Vegetable Production Guide (pp. 122-123 in 2017 Guide) for recommendations. Options are limited for organic growers and gardeners, although row covers until flowering will protect plants when they are most vulnerable.

Brown longitudinal lesion and root rot of eggplant caused by southern blight; white mycelium of the pathogen Sclerotium rolfsii

Southern blight. We found this disease on young eggplant and pepper seedlings on OSU’s Waterman farm in Columbus. This is a disease that thrives under hot, moist conditions – it is not common in northern Ohio but we have seen it before in central and southern Ohio. An elongated brown lesion on the lower stem and root rot were accompanied by white mycelium (see photo), sometimes clearly in a fan shape, at the base of the stem. Small, round sclerotia eventually form on the surface and serve as overwintering structures. Sclerotia can survive many years in the soil, so it is important to remove diseased plants and accompanying sclerotia from the field.

Lesions of Septoria blight on tomato seedlings

Septoria blight of tomato. The disease was diagnosed tomatoes from Highland County. Typical symptoms are small necrotic spots on leaves, but not on fruit, which can help distinguish this disease from bacterial spot. Necrotic lesions caused by Septoria lycopersici contain tiny round black structures called pycnidia that can be seen with a hand lens. The pycnidia contain spores of the pathogen that are released when exposed to water. Therefore the disease is rare in greenhouses and high tunnels unless there is exposure to rainsplash or overhead irrigation. Septoria can be seedborne – we diagnosed Septoria blight in tomato transplants from a greenhouse with overhead irrigation earlier this spring in which infested seed was a likely source. Septoria blight is best managed with timely fungicide applications; a strobilurin fungicide should be included in the program.

Severe 2,4-D damage on tomato

Herbicide drift damage – high tunnel tomato. We have seen several cases of herbicide drift damage to tomatoes from high tunnels so far this season. When we receive samples that look like possible herbicide damage, we pass the sample on to OSU’s Dr. Doug Doohan. He diagnosed the sample pictured here as damage from exposure to the herbicide 2,4-D. This was a very severe case.

Corky root rot of tomato

Corky root rot of high tunnel tomato. This disease is often part of a soilborne disease complex of tomato that may also include Fusarium wilt, Verticillium wilt, black dot, and root knot nematode, among others. In this case, corky root rot was the primary problem. Like other soilborne diseases, corky root rot is very difficult to manage if resistant varieties are not available. Where the disease is a problem, grafting onto a corky root rot- resistant rootstock is very helpful in managing the disease. We are also working on anaerobic soil disinfestation (ASD) as a method of reducing populations of soilborne plant pathogens in high tunnels.

Many thanks to OSU Vegetable Pathology lab members Dr. Francesca Rotondo, Dr. Anna Testen and Claudio Vrisman for their work on these cases.

High Tunnel/Greenhouse Tomato Diseases Starting to Appear

While open-field tomatoes are just getting started in parts of this region, high tunnel and greenhouse tomatoes have been in the soil for quite a while now to get that early start on the season. So we are seeing a number of diseases appearing on tomatoes produced in high tunnels and greenhouses. Some of the diseases are soilborne, some airborne, and some spread by insect pests. Here are some examples we’ve seen so far this spring:

Tospoviruses – Tomato spotted wilt virus (TSWV) and Tomato chlorotic spot virus (TCSV). Symptoms include yellow and/or necrotic spots on leaves, as well as purplish-to-brown spots or streaks on petioles and stems. Plants may wilt and new growth may be very stunted. Fruit symptoms appear as blotchy yellow or necrotic spots or ringspots, depending on the virus. Tospoviruses are transmitted by thrips, which acquire the virus while feeding as larvae on infected plants. Adult thrips do not acquire these viruses, although thrips that are infected retain the virus for their lifetimes and can pass the virus on to their progeny. We can test for TSWV quickly with a commercially available immunostrip test, but TCSV requires a more time-consuming lab test. Once the disease is observed in a greenhouse or high tunnel, the infected plants should be removed and destroyed, or they will serve as a reservoir of the virus. Thrips populations must also be controlled. More information can be found in last year’s post on TSWV in peppers and tomatoes and on OSU’s Vegetable Disease Facts website.

Botrytis gray mold. This is a disease caused by the fungus Botrytis. Botrytis spores are windborne and are most likely to cause problems when the weather is cool, skies are overcast, and humidity in the greenhouse or high tunnel is high. Leaves, stems, petioles and fruit can be infected. Fuzzy brown-to-grey moldy growth on infected tissue is the telltale sign of grey mold disease. The disease is managed by cultural practices such as reducing humidity in the structure by ventilation, pruning plants adequately, maintaining fertilization, especially calcium, at a level optimal for plant growth, and removing dead and dying tissue to reduce the spore load. Several fungicides are labeled for use on tomatoes in greenhouses or high tunnels, including Fontelis, Botran, and Scala. See the Midwest Vegetable Production Guide for more information.  Products such as Oxidate, Fracture and various biocontrols may reduce disease pressure and be acceptable for organic growers as well. See our Botrytis gray mold factsheet for pictures of symptoms and an in-depth discussion of disease management.

Pith necrosis.  Pith necrosis is also a disease favored by cool weather, so it is more of a problem in the spring and early summer in tomatoes produced under protected culture than in mid-summer.  It usually appears to be randomly distributed in the tomato crop, and is associated with high nitrogen in soil.  Pith necrosis is caused by several species of bacteria, most commonly Pseudomonas spp., all of which are common soil inhabitants.  Affected plants may wilt and die; and brown to black lesions may be seen on the outside of the stem or petioles.  The bacteria can be spread from plant to plant through contaminated tools or hands, so sanitation is very important in the management of the disease. Diseased plants should be removed from the greenhouse or high tunnel and destroyed. Pith necrosis can be diagnosed by splitting the stem lengthwise with a sharp knife and checking the pith in the center of the stem. Diseased pith is dark brown in color. You can find more information and pictures of symptoms on our Vegetable Disease Facts-Pith Necrosis website.

Verticillium wilt. This disease is caused by the fungus Verticillium, which lives in soil and infects plants through the roots. The disease is not usually a major problem in tomatoes in open fields as long as adequate crop rotation is practiced and resistant varieties are used. Adequate crop rotation (out of tomato/pepper/eggplant for at least 3 years) is not always practiced in greenhouses and high tunnels, so we are seeing Verticillium wilt and other soilborne diseases emerging in these tunnels. Further, many heirloom varieties popular for high tunnel/greenhouse production are very susceptible to Verticillium wilt. There are no effective fungicides available to control this disease. We are working to adapt a relatively new approach called anaerobic soil disinfestation (ASD) to protected culture systems, which has the potential to reduce disease pressure caused by Verticillium and other soilborne pathogens. (Verticillium photos by A. Testen).

White mold. This disease is caused by the fungus Sclerotinia, and is most common under cool, wet, high humidity conditions.  See my post a year ago on white mold management in high tunnel tomatoes for details.