Black rot on cabbage, caused by the bacterium Xanthomonas campestris pv. campestris. Note characteristic “V” shaped lesions starting at leaf margin. Photo: Lee Stivers, Penn State
Weather conditions are causing some strawberry crops to ripen slowly. Not surprisingly, we have observed several strawberry diseases around the state, including common leaf spot, anthracnose, and botrytis blight. There have also been reports across the Northeast of cyclamen mites in strawberries. In matted-row plantings, an effective time to treat for cyclamen mites with miticides is after mowing at renovation, as the majority of the mites reside at the base of the leaf petioles in the crown area.
Striped cucumber beetles are now active in many vine crop fields in Pennsylvania. Not only do these insects directly damage young seedlings, flowers and immature fruit with their feeding, they also vector bacterial wilt disease, primarily to cucumber and muskmelon, but sometimes also to pumpkin and squash.
While cucumbers in Pennsylvania are not yet at risk, there are several reports of downy mildew on cucumber as far north as North Carolina. Alternaria has been observed on several vine crops in the state so far this season, including cantaloupe seedlings and cucumbers. Angular leaf spot is also showing up on cucumbers, particularly in local varieties where the seed is saved year to year.
There have been several reports of root and stem rots in newly emerging sweet corn plantings. While the specific pathogens have not yet been identified, a number of fungal root pathogens can infect corn seedlings, such as Fusarium, Diplodia, Pythium, and Macrophomina. These fungi infect root and stem tissue, causing stunting, and often, seedling death. Sometimes the best option is replanting the field. Pheromone traps for monitoring lepidopteran pests of sweet corn (European corn borer, corn earworm, and fall armyworm) are being set out around the state; reporting of trap counts will begin the week of June 18.
Late blight (Phytophthora infestans) is, of course, a concern in a growing season with lots of rainfall. There is one report of tomato plants shipped to a garden center in northeastern Pennsylvania that were infected with late blight; these plants were destroyed.
Early varieties of tomatoes in high tunnels are developing heavy fruit loads, although some problems with flower abortion due to the cold, wet weather have also been observed. Magnesium deficiency, a fairly common occurrence, is also starting to appear. Magnesium deficiency symptoms include interveinal chlorosis (yellowing) and curling on the lower leaves. Tomatoes typically scavenge magnesium from the lower leaves to meet the demand of the maturing fruit. While not usually an acute problem, it can be addressed through foliar applications of magnesium (for a quicker fix; 1 lb Epsom salts per 100 gal water) or through the drip irrigation system (greater crop safety; at a rate of 20 lb Epsom salts per acre). While only low levels of bacterial spothave been observed in the high tunnel and greenhouse tomatoes, botrytis grey mold has been much more frequent due to the cold, wet conditions.
Other observations around the state include black rot in cabbage, possible Rhizoctonia stem rot in lettuce, flea beetleson cabbage, and asparagus beetlesin asparagus. In the Penn State onion trial at SEAREC in Landisville, purple blotch is starting to develop. Poor potato emergence has also been observed in select fields due to soft rot issues.
Symptoms of magnesium deficiency in tomatoes include interveinal chlorosis and leaf curling on lower leaves. Photo: Lee Stivers, Penn State