CROP DISEASE UPDATE FROM DR. BOB KEMERAIT, UNIVERSITY OF GEORGIA EXTENSION PATHOLOGIST
This Memorial Day Week is an EXCELLENT time to both assess disease management strategies in your row crops and to assess the risk for the diseases themselves. The importance of this week is based on two critically important factors- crop development and the passage of Tropical Storm Beryl. Even those growers who did not receive a drop of rain from Beryl should still consider disease management now- if for no other reason than winds from the storm may have moved fungal spores over long distances and dropped them literally in their laps.
Whether you have corn, cotton, peanuts, or soybeans in the field now and received some rainfall and cloudy conditions from Beryl, then the risk of fungal diseases in your field increases. One factor is that the rain and subsequent cloudy weather, coupled with high humidity, have created near-perfect conditions for the formation of spores on existing spots and lesions. Also, raindrops hitting last year’s crop debris could also send spores to infect lower leaves of the crop. A second factor is, obviously, that the spores that detach from the spots and lesions and land on other leaves also have near-perfect conditions for infection to occur. Extended periods of leaf wetness significantly increase the chance that a spore that lands on a leaf (or other suitable tissue) will infect. A third factor has been the wind. On the level of a field, the mechanical action of leaves brushing up against one another coupled with the mechanical dispersal of rain drops hitting the leaves will send spores from one leaf to another and perhaps beyond. Even if you didn’t receive a single drop of rain from Beryl (wow….) your crop is still at risk from long spread dispersal of spores like southern corn rust and Asian soybean rust. For example, rust spores could be carried in upper air currents from Florida and deposited on the crop; cloudy weather reduces UV radiation and further increases the potential of the spores to survive over long distances. Bottom line: I believe that it is very likely that many fungal infections developed in our crop over the past 48 hours; a week from now we may very well begin to find outbreaks of diseases that would have been much delayed had Beryl not blown our way.
So what to do? The first thing I will recommend is for every row crop grower in the path of Beryl to consider what benefit a fungicide could provide at this time. NOTE- I DO NOT SAY that every corn, cotton, peanut or soybean grower should spray! Simply, conditions are now favorable for the spread of important diseases of each crop. Below are some examples:
Peanut- If your crop has not reached 28 days after planting, I would not be too worried about leaf spot diseases, unless you are planting peanuts behind peanuts. A short peanut rotation will increase the risk for early outbreaks of all diseases. White mold? Yes- that could be a problem early this year. Warm temperatures early in the season coupled by an extended period of moisture could further ignite a white mold epidemic. Bottom line for peanuts? I probably would not change much in an already sound peanut fungicide program, other than to perhaps initiate the program a bit earlier. And where leaf spot disease is already in a field, consider use of a fungicide with curative activity (Headline, Tilt/Bravo, Stratego, etc.) over a pure protectant like chlorothalonil alone. Last couple of notes on peanut: A) so far, tomato spotted wilt has not seemed to be a bad as I thought it might be- growers are doing an EXCELLENT management job! B) Cooler, wetter weather will certainly help to reduce the threat of Aspergillus crown rot on young seedlings.
Corn- Much of the commercial corn crop has reached tasseling or will do so quickly. I believe that “tasseling” is an important growth stage where growers need to consider the need for a fungicide; tasseling is NOT the time for an “automatic” application for each and every grower! For example, I believe that some small amount of northern corn leaf blight can be found in the majority of fields in the state but this alone does not necessitate a fungicide application. Short corn rotations, a more susceptible hybrid, and wet, windy weather will increase risk to the disease and the grower should be prepared to make an important decision. It has been my experience that unless a hybrid is especially susceptible to NCLB, this disease does not “explode” in a field like rust does. Still, after a passing storm and extended leaf wetness periods, the disease could develop quickly in fields at risk. In such cases, TIMELY applications of a fungicide with curative activity (triazole fungicides) will be more beneficial than a protectant fungicide alone. Benefits of a triazole/strobilurin mix? Extended protective window and curative activity. Bottom line: with Beryl, conditions for diseases in corn are enhanced in south-central and southeastern Georgia. This simply further increases risk; an increase that growers should consider in their decision to spray (or not). If a grower has sprayed a fungicide and is considering an additional spray, he or she should assess the level of disease in the field prior to the storm and the time since the last application. FINALLY- With the passing of Beryl, we will continue to closely monitor the potential for introduction of southern rust with the winds.
Cotton- Where rainfall was abundant (it wasn’t in Tifton) growers could see some temporary increase in seedling disease. Also, the same wet weather could produce outbreaks of Ascochyta “wet weather” blight which also should be of only temporary importance. I am most curious about the impact of the weather on Corynespora/target spot. In fields where the disease has occurred in the recent past, the falling rain could splash the fungal spores from the soil to the leaves of even young plants. Will this happen in 2012? I don’t know but I strongly advise growers who find spots on their cotton at any growth stage approaching first square (and beyond) to have it diagnosed quickly. As always- the best way to manage a disease like Corynespora/target spot is to protect the crop early before it becomes established.
Soybeans- Asian soybean rust is known to be active on kudzu just across the Georgia-Florida line in Leon and Gadsden Counties. It has not yet been found in Georgia; however I expect it will be sooner rather than later. Once the disease is found, soybean producers in surrounding areas should protect their crop by the late bloom stage. As in southern corn rust, UGA Cooperative Extension is monitoring the introduction and spread of soybean rust closely and results can be found at www.sbrusa.net.