Koror, Palau
Identification request
Dying Zucchini Plants

Can anyone tell me what is killing my zucchini plants?  They looked fine a week ago, but now have white spots on stem and leaf petioles and two have died completely.  The root systems of the dead plants looked fine.  Please see attached images.



Hi Joel

The symptoms on the zucchini look very much like crown rot/basal stem rot caused by the Basidiomycete fungus Sclerotium rolfsii. The large amount of undecomposed organic matter also visible in the photograph would support this pathogen and predispose the plants to infection.


Lynton Vawdrey 


Hi Joel

Agree with Lynton that Athelia (Sclerotium) rolfsii is a possibility, although you say that the roots look healthy. There are two things here. It's best to dig up the roots carefully on still living but wilted plants, and then look at the fine roots. Put the entire root system in a bucket of water, shake carefully to remove the soil and until you can see the fine roots clearly and tell if they have black tips. If they have, this is indicative of many kinds of infections, particularly Pythiums and the like. The other thing, if it's Athelia, you should see the tiny mustard seed (or smaller) size white balls of the sclerotia. These later turn brown. They are the resting structures of the fungus and last many years in the soil. I don't see any amongst the white mycelium over the stem in your photo. 

My guess is that you do have a root rot. But it's a good thing to have a look around the stems and lower petioles to see it there are any holes or other damage that may be caused by insects, and hunt around in the soil for anything suspicious. 

The last thing is what was grown in the soil before the zucchinis?

Lots of articles on the internet describing root rots of zucchinis; there's one: https://homeguides.sfgate.com/zucchini-root-rot-prevention-37517.html

All the best


Thanks.  I'll replant on hills or ridges and keep soil bare around the base of the plants - and cross my fingers.



I had overlooked the white spots on the stem and petioles, although now I see you mentioned them in the original message. Now, I am no squash nor pumpkin expert, but these plants have juicy tender vines, and I wonder if you have a green vegetable bug, or another stink bug, or perhaps even a plant footed bug, piercing the stems, and then there are secondary infections. Can you have a look for anything large with long proboscis. There are fact sheets in the app covering insects of the kind.

I take it that there are no white patches on the leaves?

Some more pics would be good.



Thanks very much Grahame, and apologies for delayed response.  The roots looked fine (plants are all dead now), and no sign of sclerotia, or of mycelium on the stem.  Just the white spots and then necrosis, so I don't think it is Athelia.  Now another variety of winter squash I ordered from the US, Sweetmeat, a vining type of squash, is showing the same symptoms.  Plants grown from seeds from local pumpkin/squash purchased at a local market are fine.  Maybe I should just stick with varieties known to be adapted to our humid tropical environment.