I have certainly seen resistance reactions of some plants to powdery mildew
which is not suited to that species. The resistance reactions can involve
plant cells dying rather than being parasitised by the fungus. There is
likely to also be some production of "secondary chemicals" (a handy term for
often complex molecules formed by organisms where the molecule is not of
direct metabolic use but is often involved in warding off pathogen and pest
damage etc). The production of any anti-fungal compounds would be induced
resistance and may reduce the infection from the proper form of mildew.
One of the many problems with taking advantage of this phenomenom would be
that you may need to find a powdery mildew form that is closely related to
one that attacks your crop plant. Generally this would be a powdery mildew
form that you've collected from a plant reasonably closely related to your
plant. E.g. for lettuce powdery mildew you could collect Erysiphe
cichoracearum spores off Calendula (same species of powdery mildew on both
lettuce and Calendula but very likely different forms i.e. formae
speciales). Erysiphe cichoracearum is reported on members of over 15 plant
families but depending on definition true members may be only present on
members of the daisy family and within this family there are many formae
speciales which to varying extents will only infect 1,2, 3 or more species
to any extent.
As Russ wrote, conidia are difficult to get in solution. It is better with
some surfactant or soap. Spores could either be collected by soaking
infected leaves (leaves can be dried first and with some species, dry leaves
can be left in a paper bag for several months and the powdery mildew spores
will be alright for use when they are needed. Spores could also be
collected dry by brushing them off infected plants.
One of the best ways to control powdery mildew without chemicals is to
regularly wet the plant surface especially with soapy water. This is
because powdery mildew although needing moisture at first for spore
germintation but thereafter the powdery mildew mycelium grows on the outer
surface of the plant and is inhibited by too much moisture. This is the
reason why powdery mildew is favoured by dry weather in contrast to other
fungal diseases e.g. downy mildews/rusts and general rots. Because of the
effect of just water and soap on reducing powdery mildews, you should try
out how effective a powdery mildew "vaccination" spray was compared to just
soap and water.
Any induced resistance would be temporary.
A better way perhaps of inducing resistance is to use a plant extract. Some
plants if dried, ground and soaked in water will provide the sort of
response you are looking for, reducing powdery mildew and other diseases.
Try Reynoutria (used to be Polygonum) sacchalinensis (giant knotweed).
I would be interested to hear of any results (or lack of) with the mildew
Dr Tim Jenkins
P.O. Box 558
>Mildews (both powdery and downy) are _highly_ specific. If you have weeds
>with a mildew, spraying the mildew will possibly cause the other weeds of
>species to become infested. However, little or no reaction will occur on
>planted crops (unless closely related to the weed). Also, powdery and
>mildew conidia (reproductive structures) are highly hydrophobic and
>difficult to get into solution.
>this would be a good idea. No harm will come to your crop, and the
>weed may succumb to mildew. . .Russ
>Frits v/d Laan wrote:
>> I would like to know anyone who tried something like this before.
>> To prevent mildew I use some organic extractions and I would like to
>> add mildew itself as a spray.
>> because most plants have different types of mildew I would like to
>> use mildew from one of the very sensitive weeds that grow and
>> allready are white from the fungus and make a thea or spray from
>> Then spray the cultivated plants with this and wait for the natural
>> reaction from the plants or from other fungi that protect the plants
>> by destroing the mildew.
>> (Vaccination principle)
>> Does someone know something about this.
>> Frits v/d Laan
>> Organic horticulture. Gouda - Netherlands
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