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How gardeners can manage downy mildew in basil

Armed with knowledge about the pathogen’s biology, I had an idea of how it might be possible to not lose my basil once again to downy mildew at my home. This destructive disease has developed on basil in my garden every summer since 2008. The pathogen produces spores that are dispersed by wind potentially long distances. With this source it is not possible to avoid a pathogen, and it is now obvious spores will be dispersed to my basil every summer.

The pathogen needs high humidity (at least 85%) or wet leaves to infect. These conditions often occur at night and on rainy days. Humidity is below 85% during the day where I live in New York. So if I kept basil plants in pots that would enable me to deny the pathogen the conditions it needs to infect by bringing the pots inside when needed (overnight and on rainy days) as humidity is never above 85% in my house.

I tried it – success!!

I brought home 6 small plants on 20 Aug, put them in larger pots, and began bringing 3 of them in most nights (sometimes forgot). The other 3 stayed on my deck to serve as “control” plants to document that the pathogen was present and conditions favorable for downy mildew to develop. The morning of 4 Sept spores of the downy mildew pathogen were observed on most leaves of the “outdoors always” control plants (these are the plants in the foreground of the first picture below). There was a lot of dew on these plants, thus conditions were clearly favorable for disease development. There were spores on the top side of some leaves which is unusual.

In sharp contrast, no spores were found on any leaves of the plants being brought indoors. (They are behind the”outdoors always” plants in the picture below.) A few leaves (4) with downy mildew were found on the “indoors at night” plants on 1 Sept, perhaps a result of having been inadvertently left outside one night when conditions were somewhat favorable.

I also put 2 basil plants in the ground in my vegetable garden as usual. They were transplanted on 9 July. At that time I put 4 plants in pots and started bringing 2 of them in every night. I saw symptoms on the garden plants on 15 Aug. They were severely affected. Interestingly, there were no symptoms on any of the plants in pots, regardless of where they were kept overnight. This might have been due to conditions being favorable for downy mildew in my garden but not on my deck in Aug. However, symptoms were not seen through early Sept when the younger plants became affected.

Throughout this period the older plants appeared stressed, likely reflecting the fact they were pot-bound. Leaves were small, yellowish, and hanging down plus lots of flowers were being produced; I tried to alleviate by removing flowers and fertilizing, but it is hard to maintain adequate soil moisture level when plants are very pot-bound.

Below: Plants that stayed outside show symptoms (leaf yellowing) and signs (sporulation) of infection. Plants in the rear were brought inside most nights.

basil downy mildew

Below: Sporulation on ‘outside’ plant (left) but not ‘indoor’ plant (right).

basil downy mildew

Below: Sporulation on both surfaces of small leaf.

basil downy mildew

Below: Sporulation on upper surface of large leaf.

basil downy mildew

Below: Sporulation on wet leaf with dew drop.

basil downy mildew

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