Fungi of the Fall - notes from Linda Gilkeson
The fungus is among us, with powdery mildew top of mind for many gardeners. That’s the white, dusty coating you might be seeing on leaves of squash, peas, and other plants. Different species of fungi cause ‘powdery mildew’ disease on different host plants. Infections spread rapidly in the late summer and fall, when dry warm days and cool night provide perfect conditions for the disease. Old leaves are more easily infected than vigorous new growth, so keeping plants growing vigorously, with lots of new growth coming on helps keep up production.
A common question is whether you should pick off leaves with PM. I don’t because until the leaves die plants are likely still getting some benefit from photosynthesis. Besides, spores are everywhere now so it doesn’t help with control. When the infected leaves turn brown and dry up you can compost them or chop them into the soil to get them to break down quickly.
My approach to powdery mildew:
1) Ignore it in some plants. For winter squash, the fact that vines are dying back now is not a problem, because the fruit should be maturing right now anyway. Peas are still producing a good crop of pods, despite some mildew—so just pick the peas, wash the pods and carry on.
2) Plant resistant cultivars. There are resistant zucchini, cucumbers, grapes, roses, etc. I get 4-6 weeks more of good production in the fall from PM resistant zucchinis than from susceptible one (some really susceptible cultivars have died by mid-September in my garden). I have had great success with ‘Partenon’ and ‘Anton’ from William Dam Seeds. www.damseeds.ca
There are sprays, of course, but you must get good coverage on both sides of leaves to be effective; two non-toxic sprays:
1) Unlike other fungi, powdery mildew spores can’t germinate when there is water on the leaves. Rinsing off all leaves (both sides) with water at mid-day, several times a week, washes the spores off and keeps new infections to a minimum.
2) Research has found that a protein in milk can inactivate the powdery mildew spores that infect cucumber and squash (not other PM fungi). The mix used was 1 part milk to 9 parts water, sprayed on leaves weekly.
Other fungicides, such as sulphur sprays or a baking soda product (e.g., Remedy TM in the US) are listed to control powdery mildews. These have to be used frequently to keep leaves covered before they become infected. They won’t cure an already established infection. Note: Frequent use of either product can damage leaves of cucumbers and squash.
Aphids on cabbage family plants, are the other problem that is common this time of year. You might not realize that the whitish-grey, stick masses in curled leaves and in broccoli heads are crowds of aphids until you look closely. These will disappear in October as the insects die off before winter. Meanwhile, I use water sprays to blast them out of the florets of broccoli and keep them out of developing Brussels sprouts and cabbage heads. I don’t want to harm the aphid predators that are also in the plants so insecticides, such as soap.
I usually don’t bother controlling aphids on winter broccoli or winter cauliflower because the aphids won’t be around when the plants are producing their crop. This year I was really fortunate to have a grand infestation of aphids in dill self-sown around the garden. The dill is full of lady beetle larvae, aphid midges and other aphid predators, so last week I cut off the dill and distributed it in and around the cabbage, broccoli and Br sprouts. Don’t worry, the species of aphids on dill won’t attack cabbage plants, but the predators do feed on other species of aphids. Checking yesterday I saw ladybeetle larvae throughout the cabbage patch so my plan is working….