Be On The Lookout For Damaging Diseases Of Tomatoes
March 1, 2016 
Septoria leaf spot causes small circular lesions that are tan in the center and surrounded by a dark margin. In the center of the lesion can be small black dots — which typically need a hand lens to be seen — that are the fruiting structures of the fungus which produce the condia (spores) the pathogen needs to spread.
Photo credit: Beth Gugino, Penn State University
Mid-Atlantic tomato growers have been faced with the perfect storm of weather conditions recently, resulting in increased rain along with a challenging set of diseases in its wake.
Beth Gugino, Associate Professor of Plant Pathology at Penn State University, answers questions about the state of late blight, bacterial spot, bacterial speck, and septoria leaf spot in the region, and provides information about symptoms and environmental factors that lead to disease spread, cultural control methods, and more.
How has the weather impacted disease on tomatoes in the Mid-Atlantic region?
Gugino: It seems like the storms we are getting are more extreme than in the recent past. Instead of getting a little bit of rain frequently, there are more storms resulting in 1, 2, or even 3 inches of rain at a time, and then periods between rain can be very dry.
Many pathogens are dispersed by rain splash and are dispersed even farther when in wind-driven rain. These types of storms tend to be more problematic when they are not quickly followed by a period of drying.
What three diseases are currently the worst facing growers in the area? Why?
Gugino: In terms of tomato production, since 2009, we have been seeing late blight (Phytophthora infestans) develop on an annual basis on both tomato and potato when previously it was every three to four years or longer between outbreaks. We are also observing an increase in bacterial diseases, including both bacterial spot (four species of Xanthomonas) and speck (Psuedomonas syringae pv tomato). Depending on the season, Septoria leaf spot (Septoria lycopersici) is also becoming more problematic.
What environmental factors cause or help each of these diseases to flourish?
Gugino: For late blight, it is a pathogen that prefers cooler temperatures, between 60° to 80°F, and wet conditions. Leaf wetness is key for late blight disease development. The wet conditions could include rain, fog, or heavy dews. Late blight can occur any time during the growing season.
In Pennsylvania, we have confirmed it in the state as early as mid-May and as late as mid-July over the past five years. Once temperatures get above 88°F disease progression will stop; however, once the temperatures drop the disease will pick back up again.
Bacterial spot is favored by warmer temperatures than bacterial speck, 75°F to 90°F compared to 63°F to 75°F, respectively. Wet conditions favor bacterial disease development, especially wind-driven rains that accompany thunderstorms, which easily spread these bacterial pathogens around the field.
Septoria leaf spot is favored by wet conditions and moderate temperatures in the mid to upper 70°s F.
What are the main symptoms of each disease?
Bacterial spot (pictured above) is favored by warmer temperatures than bacterial speck; 75° to 90°F compared to 63 to75F, respectively.
Photo credit: Beth Gugino, Penn State University
Gugino: Late blight symptoms can develop on the leaves, stems, and fruit. The lesions are initially small and water soaked and then quickly become large, irregularly-shaped, gray-brown lesions, sometimes surrounded by a light-green halo. Dark-chocolate-brown elongated lesions can develop on the stems, and on the fruit the lesions are greasy brown in color and remain firm.
Bacterial spot and speck symptoms can develop on all aboveground parts of the plant. The lesions are small brown to black spots that can sometimes be surrounded by a yellow halo. They can coalesce together and cause the entire leaf to blight down. On the fruit, bacterial spot causes a larger more corky type of lesion compared to bacterial speck, which as the name implies looks more like a speck.
Septoria leaf spot causes small circular lesions that are tan in the center and surrounded by a dark margin. In the center of the lesion can be small black dots — which typically need a hand lens to be seen — that are the fruiting structures of the fungus which produce the condia (spores) the pathogen needs to spread. Unlike early blight, Septoria leaf spot does not produce any fruit symptoms, but the disease can very quickly defoliate the plant, impacting fruit yield, quality, and marketability.
When should scouting begin?
Gugino: Late blight can affect a plant at any growth stage, so knowledge of where late blight has been confirmed in your surrounding region can help guide scouting. Checking with your local university Extension team and utilizing resources like the USAblight.org website can be important tools. So far, there have been no reports of late blight in Florida, which is a good sign.
Due to the challenges of managing both bacterial spot and speck, growers should scout from transplant production throughout the entire season. I tell growers they should assume that they are going to have problems with these diseases and take all of the preventative steps possible to reduce the potential likelihood.
I usually start scouting for Septoria leaf spot with the onset of fruiting and focus on the lower most leaves which are most susceptible because the plant is diverting resources from these lower leaves into fruit production.
What chemical controls can be used for each disease?
Gugino: For late blight there are a lot of very effective conventional fungicides, and even protectants like chlorothalonil can be very effective if they are applied in a timely manner and get good coverage.
It is more challenging to manage organically. Copper-based fungicides tend to be the primary tool along with products like Regalia (Marrone Bio Innovations) and Actinovate (Monsanto). Initiating preventative applications is critical in an organic system.
Growers are very limited in available in-season tools for bacterial spot and speck. Most of the management needs to focus on prevention. However, during the season growers can suppress disease progression with applications of copper-based fungicides tank-mixed with mancozeb.
In some trials, products like Tanos (DuPont) and Regalia have reduced disease severity. As of last year, many states in the Mid-Atlantic region has a section 2ee label for the application of Quintec (Dow AgroSciences). Another option is acibenzolar-S-methyl which is the active ingredient in Actigard (Syngenta). This product activates the plant’s natural defenses; therefore, it must applied starting at transplanting rather than waiting until symptoms are observed in the field.
In some growing regions, there are issues with copper resistance within these bacterial populations. That is something that my research group is currently looking at with financial support from the Pennsylvania Vegetable Growers Association and Pennsylvania Vegetable Marketing and Research Board.
There are a number of fungicides effective for the management of Septoria leaf spot; however, scouting and timing their applications is critical. See the 2016 Mid-Atlantic Commercial Vegetable Recommendations (http://bit.ly/1PpL4Cn) for a list of recommended products.
What are the cultural controls for each disease?
Gugino: In general, employing practices that help reduce high relative humidity and leaf wetness within the crop canopy can help reduce disease development.
For bacterial diseases, sanitation is critical. These bacterial pathogens can be associated with the seed so growers should do the following:
- Employ seed sanitation practices, such as hot-water seed treatment (112°F for 25 minutes);
- Make sure greenhouse production facilities are clean (sanitize greenhouse surfaces, eliminate all weeds and other organic debris);
- Use new flats or make sure reused ones have been cleaned and sanitized;
- Scout transplants carefully and rogue entire plants if symptoms are observed;
- In the field, start with new stakes each season. Often this is not an option, so if stakes must be reused growers should take measures to clean and sanitize them by power washing and soaking in a disinfectant.
In talking with growers last season, there is interest in the possibility of heat treating stakes due to access to kilns across Pennsylvania. Wet heat (steam) is usually more effective, but this is something we are interested in looking at more. Unfortunately, cold temperatures are less likely to kill these pathogens, so leaving the stakes outside overwinter won’t be enough.
Aside from late blight, which is an obligate pathogen (requiring living plant tissue to survive), the bacterial pathogens and Septoria leaf spot are associated with crop residue. Utilizing three- to four-year rotations that allow the crop residue to thoroughly decompose will reduce the pathogen population.
In terms of host resistance, there are some commercially available late blight resistant cultivars. These include Iron Lady from High Mowing Organic Seeds and four varieties from Johnny’s Selected Seeds: Mountain Merit, Mountain Magic, Defiant, and Plum Regal. In recent trials, cultivars that contain two resistance genes are holding up even under very high disease pressure.
What can growers do now to get a jump on protecting their tomato crops in the coming season?
Gugino: Recognize that the primary strategies for vegetable disease management are proactive and preventative (site selection, crop rotation, soil health, sanitation, cultivar selection, pathogen-free seed, and transplants, etc.).
Knowledge is the key to success. It’s important for growers to be familiar with the diseases they are likely to develop on their crop as well as the symptoms and signs. Also, be familiar with the basic biology of the pathogen (how does it spread, survive, overwinter, etc.) and the environmental conditions that favor disease development. Be familiar with management practices and strategies.
It’s also important for growers to keep good field notes that detail what diseases they were having trouble with, on what cultivars, and in what fields. This information can be used to help decide on cultivars, and modify crop rotations.
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