In an update to my previous post, southern corn rust has now been confirmed in Chesapeake, VA. This is a few days earlier than in 2014 (August 3), but most of the field corn in the region is mature enough (dent stage) that yield should not be impacted. Late planted corn in fields with good yield potential (120 bu/ac +) may need to be protected with a foliar fungicide. Strobilurins are good preventative fungicides whereas triazoles are recommended once sporulation is observed in a field due to their curative activity. A combination fungicide is a good option as long as a strobilurin fungicide has not been applied previously. Wet, warm weather favors disease development. Once sporulation occurs, symptoms of rust are relatively obvious. Lesions start out as raised, blister-like pustules then break open to reveal orange spores. Samples of corn plants with symptoms of southern rust can be submitted to the disease clinic at the Virginia Tech Tidewater AREC (contact Dr. Hillary Mehl, hlmehl@vt.edu).
Category Archives: Disease
Corn disease update – July 26, 2015
Much of the field corn in the region is near or at the dent stage and no longer at risk for yield loss from foliar diseases. One disease I frequently receive questions about is southern corn rust. Southern corn rust is a potentially aggressive disease, but the fungus does not overwinter in Virginia and it is typically seen late in the growing season if at all. In 2014, southern corn rust was confirmed in Virginia on August 3, which is relatively early compared to other years. As of this week, southern corn rust has been confirmed from four North Carolina counties (Hyde, Lenior, Beaufort, and Camden) but it has not been observed in Virginia. We very well may see southern corn rust in southeastern Virginia within the next couple of weeks, but there is no need to panic. Yield of corn at or near the dent stage is unlikely to be impacted by the disease, but late planted corn in fields with good yield potential (120 bu/ac +) may need to be protected with a foliar fungicide. Strobilurins are good preventative fungicides whereas triazoles are recommended once sporulation is observed in a field due to their curative activity. A combination fungicide is a good option as long as a strobilurin fungicide has not been applied previously. Wet, warm weather favors disease development. Once sporulation occurs, symptoms of rust are relatively obvious. Lesions start out as raised, blister-like pustules then break open to reveal orange spores. Samples of corn plants with symptoms of southern rust can be submitted to the disease clinic at the Virginia Tech Tidewater AREC (contact Dr. Hillary Mehl, hlmehl@vt.edu).
Southern corn rust
Fungicide Resistant Frogeye Leaf Spot Present in Virginia
Fungicides are an important and effective tool for management of fungal diseases of crops including soybean. Unfortunately, over time fungal pathogens have the potential to develop resistance to specific fungicide modes-of-action. Mutations conferring resistance to fungicides are relatively rare, but multiple applications of the same fungicide chemistry impose selection pressure on pathogen populations and increase the frequency of those mutations over relatively short periods of time. Thus, specific fungicide chemistries have the potential to lose their effectiveness over time. Fungicide resistant isolates of Cercospora sojina, the causal agent of frogeye leaf spot in soybean, have recently been confirmed throughout the southeast including in North Carolina in 2013 and Virginia in 2014. In 2014, a small preliminary survey was conducted to determine if fungicide resistance is occurring in Virginia populations of frogeye leaf spot. Four fields were tested, and two of those fields had fungicide resistant strains. The resistance is specific to strobilurin (QoI, FRAC group 11) fungicides, which are highly effective for control of fungal foliar diseases but to which resistance can rapidly occur within fungal populations. Effective foliar disease management requires appropriate fungicide chemistry selection based on the specific pathogens present and their sensitivity to different fungicide modes of action.
Additional isolates of the frogeye leaf spot pathogen from throughout Virginia need to be collected and tested for fungicide resistance so that appropriate fungicide recommendations can be made. Other states have already initiated fungicide resistance monitoring programs for the causal agent of frogeye leaf spot (C. sojina), and we will implement a similar program in Virginia. Soybean leaves with symptoms of frogeye leaf spot will be collected throughout the 2015 growing season, the fungus will be isolated, and isolates will be tested for resistance to strobilurin (QoI) fungicides. If fungicide resistance is widespread in Virginia, recommendations for foliar fungicides and/or cultivar selection may need to be modified. If incidence of fungicide resistance is low, we will continue to monitor fungal populations and assess the risk of fungicide control failures on a year-by-year basis.
In order to implement an effective fungicide resistance monitoring program, we are requesting that leaf samples from soybean fields with symptoms of frogeye leaf spot (see pictures above) be submitted to the Tidewater AREC for testing. For more information on submitting samples, please contact Dr. Hillary Mehl (757) 657-6450 ext. 423 or hlmehl@vt.edu.
Peanut Disease Update
Recent warm, wet weather has been optimal for both plant growth and fungal disease development in peanut and other crops. Early planted peanut in southeastern Virginia is near the R3 (beginning pod) stage, so it is almost time to make the first fungicide application for leaf spot. The first spray can be delayed until two weeks after R3 on Bailey. Last effective spray dates for leaf spot can be found on the Virginia Peanut-Cotton Infonet website.
Sclerotinia blight risk in based on temperature, rainfall/humidity, and crop growth. Temperature and rainfall have been conducive for Sclerotinia blight, but Sclerotinia risk is considered low to moderate prior to canopy closure. However, rapid growth of the peanut crop in some fields has resulted in thick canopies that provide the moist micro-climate necessary for disease development. Thus, now is the time to start scouting for Sclerotinia blight in peanut fields, especially if vines are within six inches of touching. If the wet weather continues, Sclerotinia blight risk is expected to be high within the next couple of weeks.
Corn Disease Update
Recent warm, wet weather has favored development of foliar diseases in corn and other crops. Northern corn leaf blight has been confirmed in southeastern Virginia over the past week, and gray leaf spot has been observed on corn in the region. Corn in much of the state is at or just beyond tasseling, and it is not too late to consider a foliar fungicide application. Several factors increase the risk of corn yield loss to foliar diseases and the chances that application of a fungicide will be profitable.
- Susceptibility of corn hybrid to disease. Varieties have a high turnover rate so check with your local extension office or seed dealer for current information on which varieties have some level of resistance to diseases in the region. Be aware of the specific diseases your hybrid is susceptible or resistant to.
- Yield potential. If yield potential is low, you do not have much to gain and fungicide applications are less likely to be profitable.
- Previous crop and cropping system (e.g. no till). Many pathogens are able to survive on crop residues. Keep in mind that some diseases overwinter on crop debris in Virginia (e.g. gray leaf spot) whereas others require a living plant host and must move in from warmer regions each year (e.g. southern corn rust).
- Crop growth stage and timing of fungicide applications. Diseases are more likely to impact yield at particular growth stages of the crop (typically during development of the grain) so timing fungicide applications accordingly is key.
- Disease pressure. Which diseases, if any, are present, and how widespread are they? Scouting and accurate pathogen/pest identification are critical components of any IPM program. Presence of a disease on lower leaves (2nd or 3rd leaf below the ear) at or near tasseling may indicate the need for a fungicide application. Yield loss can occur if diseases reach the ear leaf prior to grain fill, but the appearance of foliar disease following the dent stage is unlikely to impact yield. A “Corn Disease Scouting and Fungicide Guide” can be downloaded below.
- Weather. Temperature and humidity greatly influence the onset and development of disease. Even if the crop is susceptible and a pathogen is present, the risk of yield loss to disease may be low if environmental conditions are not conducive pathogen growth and reproduction. Warm, humid conditions are favorable for many diseases in our region. In some cases, the micro-climate within a field may be conducive for disease development even when ambient conditions are relatively dry, especially when high plant populations and a dense canopy are present in a field.
Corn Disease Scouting & Fungicide Guide
The Virginia Peanut-Cotton Infonet has moved (back)
The Virginia Tech Tidewater AREC (TAREC) maintains the Peanut-Cotton Infonet which provides growers in the region with daily weather data (e.g. temperature, rainfall), peanut leaf spot and Sclerotinia advisories, peanut heat units, cotton degree days, and a frost advisory. Four weather stations located in Suffolk, Capron, Skippers, and Waverly are maintained by the TAREC Plant Pathology program. These weather stations transmit data to a computer at the Tidewater AREC, and the data is used to run fungicide advisory models. Data are available on the Virginia Peanut-Cotton Infonet website (http://webipm.ento.vt.edu/cgi-bin/infonet1.cgi).
The data and information available here include:
Maximum, minimum, and average air temperatures
Average soil temperature at a 4 inch depth
Daily and accumulated (from May 1) peanut heat units
Daily and accumulated (from May 1) cotton degree-days
Daily and total seasonal (from May 1) rainfall
Last effective spray date for peanut leaf spot
Sclerotinia blight risk
Frost advisory (from September 25th to completion of harvest)
In addition to the data provided on the Infonet, current information on peanut diseases in the region and disease management recommendations will be provided here on this blog, so be sure to check back for updates.
Late blight found in NC
Late blight has been confirmed in North Carolina on potato. Few details are known right now regarding the intensity of this outbreak, but, it is focused around the Elizabeth City, NC area. Potato and tomato growers, particularly those in the SE part of the Commonwealth should be diligent about scouting their crops and consider protective fungicides. Please let us know if you have further questions and we will continue to update everyone as we learn more.
Virginia Peanut-Cotton Infonet Update
The Virginia Tech Tidewater AREC (TAREC) maintains the Peanut-Cotton Infonet website (webipm.ento.vt.edu/infonet/) which provides growers in southeast Virginia with daily weather data (e.g. air and soil temperatures, rainfall, relative humidity), peanut leaf spot and Sclerotinia advisories, peanut heat units, cotton degree days, and a frost advisory. Four weather stations located in Suffolk, Capron, Skippers, and Waverly are maintained by the TAREC Plant Pathology program. Thanks to funding from the Virginia Peanut and Cotton Boards, we have been able to upgrade our equipment which should improve the overall reliability and consistency of data transfer from our weather stations. The new equipment has been installed and we are receiving data, but we are temporarily unable to post the data to the Infonet website. This technical issue should be resolved by next week, but in the meantime daily weather data will be sent out as an email to extension agents and posted to the Virginia Ag Pest Advisory Blog. We apologize for the inconvenience, and we are working hard to get the Peanut-Cotton Infonet website up and running for the season. If you have any questions or concerns feel free to call the TAREC Plant Pathology lab at (757) 657-6450 ext. 428 or email Dr. Hillary Mehl (hlmehl@vt.edu).
Please find below the current weather data for Suffolk, Capron, and Skippers. The Waverly data will be available shortly.
Be prepared for scab in small grains
Wheat in the southeastern portions of Virginia will start flowering soon if it has not already. Wheat is susceptible to scab (Fusarium head blight) from early flowering until approximately 10 days after first flower. An online scab risk assessment tool (www.wheatscab.psu.edu) provides information on relative risk of wheat infection by the scab fungus which also has the potential to contaminate grain with DON (vomitoxin). The risk model is based primarily on relative humidity during the 15 days prior to flowering since high moisture favors inoculum production by the scab fungus on crop residues. However, the model also takes into consideration variety resistance to scab. No wheat varieties have complete resistance (immunity) to scab, but commercially available varieties vary in their susceptibility. Current variety rankings for scab resistance can be found here:
Wheat variety rankings for scab resistance
If you do not know the relative susceptibility/resistance of your variety, it is probably best to err on the side of caution and assume that your wheat crop is susceptible or at least moderately susceptible to scab. Currently, the predicted scab risk for wheat that is flowering in Virginia is low for all but the most susceptible varieties. Dry weather is predicted after the current storm passes through, so scab risk will likely remain low to moderate over the next week. However, it is prudent to be prepared to apply a fungicide for scab management as the wheat crop approaches the flowering growth stage. Keep in mind the best management tactic for minimizing yield and quality losses to scab and DON is to plant resistant varieties and to make a properly timed fungicide application when wheat is in the susceptible growth stage (flowering) and weather conditions favor dispersal of the scab fungus.
If a fungicide is applied for scab, strobilurins should NOT be used as they may increase DON (vomitoxin) contamination of the grain if applied after flag leaf. Foliar diseases are starting to increase in some fields in Virginia, but if the wheat is at or near flowering, a strobilurin or premix fungicide containing a strobilurin is not recommended. Fungicides recommended for scab also provide very good to excellent control of common foliar diseases of wheat (see last week’s post), so a scab fungicide application may provide the added benefit of controlling leaf spots and rusts. The triazole fungicides Prosaro, Proline, and Caramba are the most effective fungicides for scab and should be applied at early to mid-flowering and up to one week later. For maximum efficacy, fungicides for scab and DON control should be applied with nozzles angled at a 30 degree angle from the horizontal using both front and back facing nozzles. This will ensure that the product is applied to the grain head rather than the leaves or the ground.
Growers, agents, and consultants can subscribe to the U.S. Wheat & Barley Scab Initiative’s (USWBSI) FHB Alert system at http://www.scabusa.org/fhb_alerts. Region-specific scab alerts that provide growers with updates during critical times are sent as emails and/or text messages.
Wheat disease update
As the wheat crop approaches flag leaf emergence and heading growth stages, it is time to start thinking about disease management. When conditions are conducive to disease development (e.g. high humidity, warm temperatures) foliar fungicide applications may be necessary to protect wheat yield and quality.
Wheat diseases observed in Virginia so far in 2015 have included viruses (which cannot be managed with fungicides), powdery mildew, and leaf blotch. Disease severity thus far has been relatively low in most areas but an increase in disease can be expected with warm, humid weather. Leaf blotch, for example, has been observed on the lower leaves where it is unlikely to impact yield, but once the flag leaf emerges, this leaf surface, which feeds the developing grain, should be protected from disease if symptoms are observed on the lower leaves and conditions are conducive to disease development. Once heading and flowering occur, the developing grain need to be protected from infection by the Fusarium head blight (scab) fungus if scab risk is high. Factors contributing to scab development and proper timing of fungicides for scab management will be discussed in a later post.
Each year plant pathologists from throughout the U.S. update the following wheat fungicide efficacy table based on the most current data. Efficacy ratings for each fungicide listed in the table were determined by field testing the materials over multiple years and locations. Efficacy is based on proper application timing to achieve optimum effectiveness of the fungicide as determined by labeled instructions and overall level of disease in the field at the time of application. Judicious use of an appropriate fungicide for the target disease can protect both yield and quality of the wheat crop.