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.
For more information about this disease and its management, please see the main webpage about basil downy mildew.
Below: Plants that stayed outside show symptoms (leaf yellowing) and signs (sporulation) of infection. Plants in the rear were brought inside most nights.
Below: Sporulation on ‘outside’ plant (left) but not ‘indoor’ plant (right).
Below: Sporulation on both surfaces of small leaf.
Below: Sporulation on upper surface of large leaf.
Below: Sporulation on wet leaf with dew drop.
No downy mildew developed on basil below that was kept indoors overnight and on rainy days. Bins facilitated moving the pots everyday. Photos taken 31 July and 6 Sept 2017.
Do the spores remain in the soil? ie. Should I replace the potting soil when putting in new basil plants?
Thanks!
Meg McGrath replies:
You do not need to be concerned about this pathogen’s spores surviving in the potting soil unless you plan to put new plants in the soil within about 2 weeks of removing the diseased plants. And if you do this, the new plants will also be at risk of exposure to the original source of the pathogen for your first plants. Downy mildews are caused by obligate pathogens which means they can only survive after their host plant dies if they are able to produce oospores. The spores (sporangia) formed in the pathogen’s fuzzy growth on leaves (sporangiophores) that are not dispersed in air currents from this growth to healthy basil leaves, only survive for a few days, longer if they are in a dark, damp area. These sporangia are dependent on chance movement by wind from where they land to healthy leaf tissue. The thick-walled oospores, which are capable of remaining dormant for a long time, form when two pathogen individuals of opposite mating type (pathogen equivalent of gender) grow together and sexually reproduce. This is not known to occur with the basil downy mildew pathogen.
All my plants are affected both in ground and in pots. Is it safe to still consume the leaves by washing the spores off of them? New growth on the affected plants have some spores but no damage-wilting-browning.
Meg McGrath replies:
There are a couple of questions to ask when deciding whether it is safe to consume any plant tissue affected by a pathogen:
1. Can the pathogen infect you? There are no plant pathogens able to also infect people. There are fungal species in the genus Aspergillus that infect plants and others that infect people, but no overlap.
2. Does the pathogen produce a metabolite that could be toxic? There are plant pathogens that produce mycotoxins which can be a serious health threat, but fortunately these are not pathogens of herbs or vegetables.
Infected plant tissue eventually breaks down and rots, often rapidly, rendering it unpalatable, like leaves that are brown due to downy mildew. But early in infection, when the infected plant tissue has not started to break down, and a toxin-producing pathogen is not the cause, the tissue is both safe to consume and often still palatable. That is the case with basil leaves affected by downy mildew that are at the stage you describe: spores present but leaves otherwise looking okay.