My friend has had a beautiful and fruitful big English walnut tree on her Petaluma, CA ranch. Unfortunately, about 1.5 years ago, the fruits started to rot. Some stayed attached to the branches while rotting, others would simply drop on the ground. At the same time, they discovered an underground water leak coming from a pipe that was going right by the tree. The leak has been fixed about 1.5 years ago and she stopped watering it completely. It’s a big old tree with deep roots. Unfortunately, the fruits continue to rot. I understand that trees have a much slower timeline and need possibly years to recover from major issues like accidental overwatering but is there something else she could do to support the tree? Is it a mistake to stop watering it completely? Any thoughts and/or advice would be very much appreciated.
Sounds like walnut husk fly. Pretty much all walnuts in California will get them if you don’t control them. Do the rotting nuts look like this?
The damaged nuts are usually still edible, just kind of a pain to dehusk.
A leak in a single spot is very unlikely to rot the roots of a big old tree like that. For that to happen the tree would need to be planted in a swimming pool and fill the pool up for weeks.
And stopping watering completely is a bad idea.
An issue like the husk fly sounds much more plausible.
A plastic tarp spread under the tree at this time of year will help prevent the larvae from burrowing into the soil to pupate. Any fallen fruit and husks should be picked up and sent to the landfill or municipal green waste, not composted onsite. These two methods will help reduce population levels, provided the neighbors don’t have walnuts and there aren’t wild native walnuts around.
There is an organic bait spray that is supposed to work well called GF-120. I don’t know how available it is for home growers, but it’s commonly used to control olive fly so most ag supply stores in the area should have it. It’s spinosad mixed with an attractant that you spot-apply onto the trunk, making it safe to use around bees.
I agree that the tree should continue to be watered. English walnuts will survive here without irrigation, but production plummets and the tree looks awful.
They need to make sure the tarp completely covers the soil under the tree canopy. It won’t reduce the amount of damaged fruit this year, but it should help reduce the number of adult flies next year.
No, there needs to be some form of bait to attract the adult flies. Supposedly molasses works as bait, so your friend could try mixing some with spinosad if they can’t get GF-120.
