Sunday, July 27, 2014

powdery mildew

the weather we've been having lately---hot days, cool nights, with a bit of humidity---has been conducive to powdery mildew growth on the squash leaves.  when i was growing squash in catheys valley they'd always get a little powdery mildew at some point when the summer really started heating up but it was never much of an issue---conditions didn't stay favorable to its growth for long and the plants would outpace it without any intervention from me.   i don't know if the weather up here is just more favorable to powdery mildew growth or if this is just a bad year but i've had to help the squash plants out a bit.   i've gone through the squash patch a couple of times and cut off the worst affected of the leaves, collecting them in a plastic garbage bag and putting them in the trash (rather than composting them).  i've also used foliar sprays to keep the situation from getting out of control.  the first spraying was with baking soda diluted in water (4 tsp/gal with dr. bronner's magic soap as a sticker) and the second spraying was with milk diluted in water (5 cups/gal).  the baking soda spray changes the pH on the leaf surface and makes it inhospitable to mildew growth.  the proteins in milk when exposed to sunlight are thought to have antimicrobial properties so i thought i'd give it a try too.  both the baking soda and the milk are best applied as preventatives but they seem to be working pretty well at keeping the situation from getting any worse and the plants are putting out healthy new growth (and we're still getting plenty of fruits) so if i keep a close eye on things i think complete disaster can be avoided.

powdery mildew on yellow crookneck squash leaf

powdery mildew compromises a leaf's ability to photosynthesize and eventually completely destroys the leaf.  if enough leaves on a plant are affected fruit production declines or stops and the whole plant can even die.

things were looking pretty bad for a while...

i did use the "wait and see" method for a while.  i like to let things resolve on their own if possible.  it became clear pretty quickly that the mildew was moving faster than the plants could handle and i'd have to do something.  

getting ready to spray

i think i would have moved a bit faster with my intervention if i had noticed the outbreak at the very beginning---many of the squash plants have what's called silvering on their leaves, a natural silver pattern on the upper side of the leaf, which can make it difficult to spot powdery mildew in it's earliest stages because the two are pretty much the same color.

healthy squash leaf showing silvering

things are starting to look pretty good again.

the yellow blotches on the leaf pictured below are areas where powdery mildew was growing but has been killed by the foliar sprays.  interesting to see how the mildew sort of bleaches out the chlorophyll.

recovering leaf

there's still some mildew here and there but overall it seems to be quite manageable at this point.

squash plant getting happy again