The leaves get a disease on them, looks perhaps like a fungus. however, I saw today a small yellow cocoon embedded in the leaf where it had turned very thin and gray. And on the back side white spots which might be eggs.
I have tried better soil, more space, not watering from above. Spraying with Safers 3 in one, and also Neem oil made no difference. This same thing also attacks sorrel..
It has been happening for a few years. It often gets better later in season. perhaps because the cocoon is finished?
Any idea what this is? And what I should do? It is so frustrating.
The plants are growing well, and then because of this grayish brown attack on the leave, I have to take off most leaves and throw them away. I don’t compost them just in case the disease lives in the soil.
There are a couple of possibilities. The most common reason for such damage is leaf miners, the small larvae of which sounds like what you found in the leaf.
The good news is that you can remove the damaged leaves and new ones will grow in their place, and your plants will improve as the summer goes on, as you have noticed previously. You can also crush the larvae in the leaves and leave the leaves in place, but removal and destruction is easier and more thorough.
Next year, till the soil early in spring to kill any overwintering miners and use a floating row cover to prevent egg laying. Note that these miners also invade chickweek, lambsquarters, nightshade and plantain, so scrupulous weeding is important. We have provided more information here.
The other possibility is a fungal disease, as you initially suspected. We have provided a full description of this problem in answer to another inquirer’s question here.
I hope your plants do survive and flourish later in the season.