Wheat and Corn Pathology Update (4/15/2016)
Trey Price, Field Crop Pathology, Macon Ridge Research Station
Boyd Padgett, Wheat Pathology, Dean Lee Research Station
At the time of this writing, most wheat in the state is at or past flowering with the exception of some later maturing varieties. We have seen issues with vernalization in a few entries in variety trials throughout the state. Simply, there was not enough cold weather to trigger reproductive development. Fusarium head blight (scab, Figures 1 & 2) has been of utmost concern to the few wheat producers we have this year. Conditions have been favorable for scab during flowering, and applications of Caramba or Prosaro using maximum rates and water volumes are recommended for management. The best control we can expect is 50%, and time will tell if applications were successful or not.
Other concerns this season have been stripe rust (Figure 3) and leaf rust (Figure 4). Conditions are currently favorable for both diseases; however, stripe rust activity is slowly decreasing and leaf rust activity is increasing rapidly. Most varieties are resistant to stripe, leaf, or both rusts, and fungicide applications are usually not necessary. In susceptible varieties, rusts are effectively and economically managed with triazole fungicide applications.
Other diseases of note have been Septoria leaf blotch (Figure 5) and bacterial streak (Figure 6). Septoria usually remains low in the canopy and does not escalate to damaging levels; however, if infections occur on the flag leaf or flag leaf -1, a fungicide application may be warranted. Most fungicides provide adequate control of Septoria leaf blotch. Bacterial streak cannot be reactively managed. Fungicides are not effective, of course, so variety selection in the fall is the primary management technique. LSU AgCenter scientists rate wheat varieties for multiple diseases at multiple locations in the state, and the results are available online (www.lsuagcenter.com), from your county agent, or your nearest research station. Bacterial streak and Septoria leaf blotch can be difficult to diagnose. Older Septoria lesions will have black spots (pycnidia) within lesions, while bacterial streak will not. Younger Septoria lesions may be indistinguishable from bacterial streak lesions; therefore, a quick diagnostic method can be used. First, cut an affected leaf section then submerge in water. Wait 5-10 minutes, and observe for bacterial streaming (Figure 7). This can be accomplished on the turn row with a pocket knife and bottled water.
It is no secret that this has been a tough year for corn so far. Soon after early planting, most producers received copious amounts of rainfall (particularly in NELA) over an extended period. Many fields were replanted because of flooding. On stands that withstood the flooding, the majority of field calls have involved corn plants that had poor nodal root development causing them to fall over (Figures 8 & 9) and stressing or breaking the mesocotyl (first true stem) in the process (Please see Dr. Dan Fromme’s post for more information on rootless corn syndrome (RCS) http://louisianacrops.com/category/crops/corn/). Most producers planted on the higher end of plant populations allowing tolerable losses due to RCS.
Interestingly, damping off (Rhizoctonia solani) was commonly observed in RCS situations where fields had been planted for at least one month (V3-V4). Over time, seed treatment efficacy declined, plants were stressed (particularly at the mesocotyl), and the pathogen took advantage of optimal environmental conditions. Classic damping off lesions were observed on the upper sections of mesocotyl (Figures 10 & 11), and the pathogen was subsequently isolated in the laboratory.
Since we have a significant number of corn acres that will be relatively late, foliar diseases, southern rust (Figures 12 & 13) in particular, will likely be a concern this year. Southern rust (SR) can be devastating if it develops early (tasseling or before) and conditions (warm, wet) are favorable for development. Scouting is key to managing this disease. Typically SR will develop low in the canopy and progress upward. Fungicides are effective on SR (Table 1). If the disease is present at or before tasseling, fungicide applications are warranted. Depending on disease severity and prevailing environmental conditions, applications could occasionally be warranted between tasseling and milk stage. Applications are rarely warranted after this stage, because the crop will usually “out-run” disease progression. Keep in mind that tasseling is the most vulnerable stage to foliar diseases. As plants mature, more defoliation can be tolerated as time goes by.
Northern corn leaf blight (NCLB) is an annual problem in Louisiana. In fact, we can probably drop the “northern” at this point. Scouting also is key to managing this disease. Similar to SR, if NCLB develops during late vegetative stages or near tasseling, a fungicide application may be advisable. Once the disease initiates, it will continue to progress for the remainder of the season. Hot and dry weather may slow NLCB progression somewhat, but with most of our acreage irrigated, temperature and moisture requirements for the pathogen are satisfied until black layer. Specific fungicide efficacy data on NCLB remains elusive; however, pooling of nationwide data indicates that fungicides are effective on NCLB (Table 1). Similar to SR, the further the crop is past tasseling, more defoliation can be tolerated.
For more information please do not hesitate to contact your local county agent, specialist, or nearest research station. Please visit our websites (www.lsuagcenter.com and www.louisianacrops.com) for the latest in field crop pathology.
Table 1. Fungicide Efficacy for Control of Corn Diseases—April 2016
The Corn Disease Working Group (CDWG), which includes many members from the mid-South including several pathologists from Louisiana, has developed the following information on fungicide efficacy for control of major corn diseases in the United States. Efficacy ratings for each fungicide listed in the table were determined by field testing the materials over multiple years and locations by the members of the committee. Efficacy ratings are based upon level of disease control achieved by product, and are not necessarily reflective of yield increases obtained from product application. Efficacy depends upon proper application timing, rate, and application method 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. Differences in efficacy among fungicide products were determined by direct comparisons among products in field tests and are based on a single application of the labeled rate as listed in the table. Table includes systemic fungicides available that have been tested over multiple years and locations. The table is not intended to be a list of all labeled products1. Efficacy categories: NR=Not Recommended; P=Poor; F=Fair; G=Good; VG=Very Good; E=Excellent; NL = Not Labeled for use against this disease; U = Unknown efficacy or insufficient data to rank product
1Additional fungicides are labeled for disease on corn, including contact fungicides such as chlorothalonil. Certain fungicides may be available for diseases not listed in the table, including Gibberella and Fusarium ear rot. Applications of Proline 480 SC for use on ear rots requires a FIFRA Section 2(ee) and is only approved for use in Illinois, Indiana, Iowa, Louisiana, Maryland, Michigan, Mississippi, North Dakota, Ohio, Pennsylvania, and Virginia.
2Harvest restrictions are listed for field corn harvested for grain. Restrictions may vary for other types of corn (sweet, seed or popcorn, etc.), and corn for other uses such as forage or fodder.
Many products have specific use restrictions about the amount of active ingredient that can be applied within a period of time or the amount of sequential applications that can occur. Please read and follow all specific use restrictions prior to fungicide use. This information is provided only as a guide. It is the responsibility of the pesticide applicator by law to read and follow all current label directions. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Persons using such products assume responsibility for their use in accordance with current directions of the manufacturer. Members or participants in the CDWG assume no liability resulting from the use of these products.