Hello Mary,

Grubs can be in the soil now. They feed on the roots of the lawn, so that you can pull the grass off the soil like it is not attached. Hopefully, he used the correct insecticide ( imidacloprid, thiamethoxam, clothianidin or chlorantraniloprole) that kills grub larvae in the immature state. https://www.canr.msu.edu/news/how_to_choose_and_when_to_apply_grub_control_products_for_your_lawn

As long as the grass is green and growing with sufficient irrigation, there is generally little difficulty from applying weed and feed at this time. Except if the lawn has a fungal disease like a patch disease. With patch disease, nitrogen feeding is not suggested.

He can have insect damage (billbugs are bad now), a fungal disease, or the lawn might have been too tall when it was mowed. When the lawn is tall, the lower blades are shaded and yellow. Then when it is cut, the green top is cut off, leaving the yellowed blades. If this is the case, the application of weed and feed should green it up.

If it is not insects or too low a cutting, he likely has a fungal disease, though it is nearly impossible to be definitive with a description of “turning yellow.” There is one fungal disease – dollar spot – that occurs on nitrogen deficient lawns. So if he did not regularly feed his lawn, low concentrations of nitrogen fertilizers regularly can help the grass grow out of the disease. Milorganite is low concentration nitrogen fertilizer that can be helpful in applying low level nitrogen to the lawn. https://www.extension.purdue.edu/extmedia/bp/bp-105-w.pdf

However, if the disease is another fungal disease like a patch disease, applying nitrogen is the opposite of what you want to do. So without a positive identification of the problem, recommending anything is not advised.

To obtain a definitive diagnosis, you may want to send a sod sample to the MSU Diagnostic Labs. Take your sample across the border of a yellow/green section to identify the cause of the yellow grass. The following website describes the services and the fee structure for analysis performed by the Diagnostic Lab.: https://www.canr.msu.edu/pestid/

This website provides directions and tips for taking a good sample:

https://www.canr.msu.edu/pestid/submit-samples/

Hope this helps.