Pythium Fungus Problems in Basil
Answered by: Conrad Richter
Question from: Jakow Grajew
Posted on: August 16, 1999

Let me consult with you about a problem we have with fungus on the bush basil (Ocimum basilicum minimum). Analysis confirms the presence of Pythium. Since this is the only plant (out of the 20 types we have) with this problem we suspect the seed is contaminated. Is there a solution to this problem ?

We have not seen the same problems in our recent seed tests. Our most recent test results (June 1999) showed germination was excellent, about 80% based on a visual assessment of the grow test boxes we run in the greenhouse. There was no evidence of disease in our tests.

Pythium is responsible for several common diseases. In some fruits it is responsible for various fruit rots. In seedlings it is very common as "damping off". Here is what Cynthia Westcott says in her book "Plant Disease Handbook (Van Nostrand, 1959):

"The damping-off organisms are in the soil, not in the seed..." She also says: "Most soils contain several species of Pythium ready to perform at optimum moisture and temperature." The way to control it is to 1) sterilize soil, 2) avoid excessive watering, or late watering in the evening or during cool times, 3) increase ventilation, 4) avoid overcrowding the seedlings by sowing more thinly, and 5) make sure trays are new or, if reused, cleaned with bleach.

I know you are thinking that because only one or two items seem to be affected then it must be the seeds. But our experience with damping-off (which *every* grower has to contend with every year no matter how good he or she is) is that it can take hold in a one or two flats and leave other flats alone. Once it takes hold in one flat it spreads very quickly within the flat and may give the appearance that there is something wrong with the seeds. Trust me, that is not what is happening -- damping-off (Pythium spp.) is very, very common and can spread quickly from contaminated soil under the right conditions, and it commonly looks like it is only affecting one lot of seeds.

There are some seed treatments you can try but they only serve to kill the Pythium fungus in the soil surrounding the seeds, and they are not applied because of any fungus on the seeds themselves. Pythium, from all reports I have seen, is not transmitted by seeds. I do not recommend seed treatment; I recommend adjusting the growing conditions listed in items 1 to 5 above.

Outbreaks of damping-off can be controlled by treating the seed flats with chemical controls such as "No Damp Off" (oxyquinoline). Cold chamomile tea as a drench in the affected seed flats also works but is not as strong.

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