Hi Everyone,

New to Garden.org and was hoping you might be able to help me diagnose a squash problem I'm having. As you can see, the leaves are turning black/dry at the edges and there is some sort of black substance/rust affecting other portions of the leaf. I apply neem oil about every 10 days (in the evenings). Would anyone happen to know what this might be? Any help is greatly appreciated. Thank you!

It looks like Gummy stem blight but... there are many strains of GS blight with varying symptoms and to make identification harder different cultivars of the same vegetable can display different foliar symptoms. Tebuconazole- and Chlorothalonil-based fungicides both work against GS blight and a host of other fungar diseases so you may want to try that.
For two years do not grow any cucurbits where those squashes are growing now, destroy all material when you pull up the plant at the end of the growing season and to err on the safe side buy you squash seeds/seedlings from a reputable source. GS blight can be both seed- and soil-borne.

One final note: do not use neem oil unless you have pests you want to get rid of and even then use it sparingly and apply it around sunset. I have been driven nuts by trying to diagnose foliar damage to several plants which ended up being spray damage caused by excessive use of neem oil.
I know it's marketed very shrewdly but it's still a pesticide (albeit one with questionable effectiveness) and should be used carefully.

Thanks for responding. I will look into Gummy Stem Blight and probably will try applying a chlorothalonil based fungicide to see if it stops/slows the symptoms. I was also wondering if perhaps it was caused by the neem oil (perhaps burning/scalding the leaves) and if I brought this on myself. Thanks again.

PG90 said:Thanks for responding. I will look into Gummy Stem Blight and probably will try applying a chlorothalonil based fungicide to see if it stops/slows the symptoms. I was also wondering if perhaps it was caused by the neem oil (perhaps burning/scalding the leaves) and if I brought this on myself. Thanks again.

Believe me, scorch damage to the leaves caused by inappropriate pesticide applications doesn't look like that. You can rest soundly about that.

I applied chlorothalonil shortly after you suggested it and that seems to have brought the issue under control. It continued to spread initially and I realized (as you said) that it was not pesticide scalding and was in fact some sort of fungal/bacterial infection. Still not sure what fungal infection though (perhaps downy mildew or some type of blight). I'll keep up with applications of the fungicide. Thanks again for the suggestion.

Downy mildew on cucurbits will start with yellow splotches on the interior of the leaf that will turn dry and reddish in a few days as the tissues die. On the back of the leaf it will look like there are splotches of reddish dead tissue.
Thankfully we haven't got it here, it's one less thing to worry about!