rapid drying of the foliage
Scouting fields to confirm the presence of
Plectosporium, applying fungicides, and plowing under crop residue after harvest.
- Switching to trickle irrigation
No pumpkin or summer squash varieties are known to be resistant to the disease, although differences in susceptibility do occur. The pumpkin varieties Sorcerer, Gold Standard and sugar pumpkins seem to be less susceptible than other varieties grown in the same fields. When scouting, look for a few white, elongated, diamond-shaped lesions on lower vines and leaf petioles.
Understanding Fungicide Recommendations
Chlorothalonil (i.e. Bravo) and strobilurin fungicides (Pristine, Cabrio, Flint, Amistar = Quadris) are the most effective at controlling Plectosporium blight. However, there are several other important factors that must be considered when designing spray recommendations, such as control of other important cucurbit diseases, resistance management, and spray coverage.
Selection of fungicides for summer squash is straightforward since this crop has a relatively short season. In most cases, two or three applications of a fungicide such as chlorothalonil (Bravo) beginning at fruit set, will control Plectosporium blight, scab, and most other important diseases of summer squash. In some years, you may need to add a material to your spray mix to improve control of powdery mildew or downy mildew late in the season.
Fungicide selection is more challenging and expensive when it comes to choosing the most effective materials for a long-season crop such as pumpkins. As well as Plectosporium blight, it is crucial to manage powdery mildew, black rot (gummy stem blight), scab, and possibly Alternaria and downy mildew. To successfully manage this pest complex with adverse weather conditions, you will need to make regular applications of a fungicide mix, with at least 40 gallons of water per acre for good plant coverage, while alternating systemic products to help prevent resistance.
Systemic fungicides provide the best control of powdery mildew because they provide protection to both the upper and lower leaf surfaces where mildew infections begin. However, systemics have one mode of action per fungicide group and tend to have more problems with resistance than contact fungicides, which provide multi-site activity against diseases. Systemic fungicide resistance can occur in a single season if the product is overused. Once a disease organism develops resistance to a systemic material, the pathogen may quickly become resistant to other products in the same fungicide group (i.e. stobilurins). As a result, newer materials that have not been exposed to disease organisms as long usually tend to work better than older products, but not for long. In contrast, many contact fungicides have been used for decades without experiencing resistance problems. Although stobilurin fungicides have been some of the most effective materials available in recent years for most pumpkin diseases (including Plectosporium), powdery mildew and black rot have already developed resistance to stoblilurins in some states, and resistance for downy mildew has occurred outside the U.S.
The best resistance management strategy to help pre