Tomatillos are closely related to tomatoes and have many of the same problems. There are many problems these plants can have and while guesses can be made, a diagnosis cannot be made from a photo. There are fungal, bacterial and viral diseases that can all affect tomatoes. Yellowed leaves can be consistent with sun scald as well. This may be a virus or even damage from herbicides used nearby that can drift to desired plants. Unfortunately, Once the plant is diseased it cannot be cured. If you would like a diagnosis, samples can be sent to the MSU diagnostic lab for a fee. You can contact them through their website https://www.canr.msu.edu/pestid/ or email at <personal data hidden>.

Even with problems, you may stilll be able to harvest fruit from your plants. Also you can look to next year to try for prevention. You can look for disease resistant varieties, Use crop rotation and do not plant anything of the same family (peppers potatoes, eggplant, sunflowers) in the same area for 3 years. Corn, beans or cabbage would be good choices to rotate with.

Adding mulch around the plants can help with weeds and diseases. Dried grass clippings are a good choice for tomato mulch. Do not use chemically treated clippings such as from a treated lawn or treated and bagged mulch.

Clean-up at the end of the year is also an important component of disease control. Remove the debris of leftover diseased plants as disease can remain in the decaying plant matter.

Here are some bulletins with further information on tomato problems that could be relevant to tomatillos as well. .

https://www.canr.msu.edu/news/use_smart_gardening_practices_to_help_ward_off_potential_tomato_diseases

https://plantvillage.psu.edu/topics/tomato/infos