We received the following images of hops last week from Steve Jensen, Rustling Bines Hop Farm. I am including them in this entry with his permission. Although symptoms on the cones like these can be caused by several plant diseases, including Alternaria cone disorder, the primary problem with these leaves and cones is an infestation of two-spotted spider mite (TSSM).
Chelsea Gordon, OSU Dept. of Entomology, confirmed the diagnosis and provided the following advice:
Infestation of cones with two-spotted spider mite (TSSM) leads to the cones being unusable to brewers. The mites decrease the quality of the cones, can cause shattering of cones during the drying process, and are themselves a contaminant. Any cones showing that type of browning are likely infested with mites and should not be sold to brewers. The effective miticides labeled for hops have relatively long pre-harvest intervals (PHIs) (most are 14-28 days). TSSM populations grow very quickly in hot and dry weather. This summer turned out to have the perfect conditions to cause TSSM headaches for many growers. Early in the summer rainy, cooler weather kept TSSM levels at lower, less detectable levels, then right when cones started to form it became hot and dry, leading to TSSM population explosions. The best thing to do when we have hot and dry conditions is to scout the hop yard thoroughly for mites. A good method for scouting is to walk your hop yard in a “W” pattern selecting leaves from 5-10 plants on each leg of the “W”, and being sure to pick leaves from various heights. The mites are found on the underside of the leaves and can be seen with a hand lens. More than two adult mites per leaf in June indicates that a miticide should be applied. By mid-July, that threshold increases to five to 10 mites per leaf. The main goal of controlling TSSM is to keep them out of the cones. As long as their populations stay low they will stay on the leaves.
Michigan State University rereleased a helpful article this year following their TSSM outbreaks: http://msue.anr.msu.edu/news/twospotted_spider_mite_numbers_up_in_hopyards