[EDITOR’S NOTE – Clemson University Extension expert Tony Melton is a regular contributor to SPW. We welcome your input as well — hit us up at firstname.lastname@example.org if there’s an article or idea you’d like to share.]
As an Extension Agent serving 11 counties in South Carolina, many times I have the dubious honor of finding and reporting new problems for growers. Well, I regretfully inform South Carolina vegetable growers and the rest of our industry that Plectosporium Blight has arrived in South Carolina.
This blight (PB) caused by Plectophaerella cucumerina was first found in Tennessee in 1988 and has become an epidemic problem in most states east of the Mississippi. Without treatment, PB has caused up to 100% loss to yellow squash, zucchini, and pumpkin.
Watermelons and muskmelon are less susceptible and cucumbers are generally resistant. In Europe, PB has caused damage to a much wider variety of cucurbit crops. So personally, while I am scouting for other diseases, I will keep a lookout for PB on all cucurbits. The host range also includes snap bean and a number of aquatic plants (growers — keep that in mind while rotating crops).
Little is known about PB’s precise disease cycle, but it is thought to overwinter in crop residue and can persist in the soil for up to three years.
Y’all thought I was just blowing smoke when I recommended at least a three-year rotation on cucurbits for all these years. PB can be spread by wind or by rain splash, but wind can spread it over long distances. Some publications state that the optimal conditions are cool, wet weather but I found it in the heat of the summer and so did the folks in Florida.
Therefore, to cover my tail when I am scouting, I will be looking for it in the cool, in the heat, and especially when it is wet. So, disease management recommendations include: a three-year crop rotation; planting in sites with good air circulation to encourage rapid drying of the foliage; switching to trickle irrigation; scouting fields; applying fungicides if necessary; and plowing under crop residue after harvest.
If you do not have a Southeastern U.S. Vegetable Crop Handbook, get one from your county agent or view it here.
On the lighter side, PB is fairly easily visually distinguished from other cucurbit diseases, controlled by the old standby reliable protectant fungicides like chlorothalonil, and the strobilurins still control it.
Lesions on the plants are small, elongated, diamond/football shaped, white-to-cream-colored areas that sometime coalesce until most of the vines and leaf petioles turn white and the foliage dies.
For us country boys, it looks like someone took a pocket knife and scraped the stems of the plants and scarred it up.
Most of the time when you spray for other cucurbit diseases, you will control blight also. However, many spring cucurbit growers in South Carolina try to grow them without any sprays.
That may well become a practice of the past if PB visits your farm.