Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
We have been getting our wheat plots set up over the past week and there certainly is a fair amount of powdery mildew inoculum out there. At this point in time powdery mildew will look like grey patches on foliage and you may see some black dots present as well. Powdery mildew needs a living host to survive. This means that when we have a warm winter or you plant extra early, there is opportunity for this pathogen to colonize tissues and hang out until the following spring. When things warm up (persistent temperatures above 59°F for 1-2 weeks), the fungus “wakes up” and then you may see some white fuzz appearing on foliage. That fuzz contains spores that can be wind dispersed to other plants, resulting in new infections. The pathogen can continue to produce spores and infect provided that the environment is conducive.
There is good resistance to powdery mildew, but some varieties, particularly those that contain the pm6 resistance gene, are not doing the job for us in the mid-Atlantic. As a result, we can see some striking varietal differences when comparing different varieties side by side (Figure 1).
Figure 1. A comparison of powdery mildew damage this spring on a resistant variety (right) and a variety containing the pm6 resistant gene, which is now susceptible to powdery mildew. Fungicides would not be recommended for controlling pm in the variety on the right, saving money, reducing risk of fungicide resistance development, minimizing fungicide effects on residue decomposition and environmental impacts, and improving yield.
When you go out and put out your first shot on nitrogen, take a second to check out your field and see if you have powdery mildew. If you do, this is an indication that the resistance is not up to par and that you may see the disease start to take off in this field when things warm up. If this is the case, you may need to consider including a fungicide in with your second shot of nitrogen. Do not apply with your first shot, as the fungus is still dormant and the plant is not producing new tissues at this point, so the fungicide will not work well, if at all.
What else should you do if you have powdery in your field? Back off on the nitrogen. Rapidly growing plants cannot produce as many defensive chemicals and rapid increases in canopy density can create a nice, humid environment, allowing the pathogen to really take off. Last season we had some individuals who mistook chlorosis due to powdery mildew for nitrogen deficiency. The additional nitrogen that was applied ended up making the powdery mildew situation worse; fungicides had no impact. If you have severe levels of powdery bad in a field now, the best thing you can hope for is fairly dry or hot weather until flag leaf emergence. Under these conditions powdery mildew is not likely to not progress to the flag leaf, which would result in additional yield impacts.
What about next year? We will have good powdery mildew data on the variety trials coming out of DE and MD. I suggest going through those and seeing which varieties are resistant to our powdery mildew populations, and avoiding the susceptible ones if possible. An added bonus for you — we have a misted Fusarium Head Blight nursery up and running again in Maryland, so we can start to provide local information to you on varietal resistance to head blight as well.
Finally, a note on fungicides for powdery mildew. You can get away with generics containing propiconazole or tebuconazole without an issue. There are several products with labels for early applications, but these are cut rates. Cut rates may result in fungicide resistance development or may not be sufficient to curtail severe powdery mildew infestations. For this reason I would suggest avoiding these in severely infected fields. Read labels for restrictions, but in severely infected fields, consider using a mid/upper range rate of the product to ensure that you have sufficient efficacy. Some of these products can heat up your mix, so be careful how many things you include in your tanks. Shoot for 15-20 gallons per acre and 300-350 um droplet size. Adjuvants should not be needed at this stage in the game. Now is also a good time to see if the results of the fall applications some of you tried are the result of uncharacteristic powdery mildew infections or due to other factors. Check your untreated areas first and look for foliar diseases.