We have an outbreak of what looks like downy mildew on coleus. They are in a warm house, 70Dand 65N, humidity has been high (raining!). We send a sample in to Oregon State Univ. to confirm. In the meantime we would like to start a spray program. What has worked? Any phyto concerns?

It will be good here to have a positive identification of the pathogen as downy mildew is typically not a concern on coleus. Powdery mildew and botyris can be a problem, however, and the symptoms and environmental conditions that allow the pathogens to thrive are the same. Please let us know when your results are back from Oregon State. In the mean time, if you have a picture of the symptoms you could post that would be helpful.

While you are waiting for results it is important to first try to control the environment. We know that this may be very difficult because of the continued rainfall you are seeing. Minimize leaf wettness by allowing for a lot of space aroud the plants to maximize airflow. Avoid leaf wettness by not watering overhead and watering early in the day so that the leaves have a chance to dry before nightfall. Provide horizontal airflow at plant height. You can also reduce relative humidity levels after irrigation by heating and allowing the moisture to evaporate up into the air, then venting with cooler,drier air. This cycle may need to be repeated several times a day to keep the relative humidity levels down during periods of rainy weather.

In terms of chemical controls, since we don't know for sure which pathogen we are dealing with right now it will be important to choose broad spectrum chemicals. Heritage has been shown to very effective for control of downy mildew and it is also labeled for botrytis and powdery mildew. If in fact it is downy mildew, a stringent chemical rotation is necessary. Here is an example of an effective program for downy mildew:

One soil drench application of Subdue Maxx should be made to begin the program.
Weekly foliar sprays could include the following: Spray 2: Mancozeb (example: Protect)
Spray 2: Stature*
Spray 3: Mancozeb
Spray 4: Aliette** and Mancozeb
Spray 5: Mancozeb
Spray 6: Strobiluron (examples: Compass or Heritage) and Mancozeb
Spray 7: Repeat the program beginning with Spray 2

If the infection is severe it will also be necessary to rogue infected material. Remove it from the greenhouse in sealed bags. Plants in close proximity to the infection should be isolated until the spray program has proven effective and there are no more signs of the disease.

Because prolonged periods of rainfall and high relative humidity can be a problem in your area any of the pathogens mentioned can be an ongoing issue so it would be a good idea to use the spray routine mentioned above as a preventative measure on all your crops.