Plectosporium Blight and New Spray Recommendations for 2005

Plectosporium blight, caused by the fungus Plectosporium tabacinum (formerly known as Microdochium tabacinum), is a destructive disease of cucurbits in New England. This disease was found in Tennessee in 1988 and has since spread rapidly throughout the eastern United States. It occurred on a single farm in Massachusetts in 2000 and on at least a dozen farms in Connecticut and Massachusetts in 2003. In the fall of 2004, after two seasons of rainy weather, it was present in every field I visited from Long Island Sound to Burlington, Vermont.

Plectosporium blight is known to cause damage to a wide variety of cucurbit crops in Europe and Asia, but the strain present in the U.S. seems to primarily damage pumpkins, summer squash, zucchini and a few varieties of gourds. In wet years, which favor disease development and spread, crop losses in no-spray and low-spray fields can range from 50 to100%. Fortunately, this disease is easily recognized and can be effectively managed.

Description and Management

Plectosporium blight is favored by cool, rainy weather. The fungus can overwinter on crop residue and can persist in the soil for several years. Plectosporium has not been reported to be seed-borne. Tiny, one or two-celled, sickle-shaped spores are formed in lesions on vines, stems, fruit, leaves and leaf petioles. Spores are dispersed by wind over long distances. Lesions are small (<1/4 inch) and white. On vines, petioles and leaf veins, the lesions tend to be diamond to lens-shaped; on fruit and leaves lesions are usually round (fig. 1, 2 & 3). The lesions increase in number and coalesce until most of the vines and leaf petioles turn white and the foliage dies. Severely infected pumpkin vines become brittle and will shatter if stepped on (fig. 4). Early in the infection cycle, foliage tends to collapse in a circular pattern before damage becomes more universal throughout the field. These circular patterns can be easily detected when viewing an infected field from a distance. Numerous fruit lesions produce a white russeting on the surface and stem that render the fruit unmarketable (fig. 5). Fruit lesions also allow for entry of soft rot pathogens that hasten the destruction of the crop (fig. 6).

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 to confirm the presence of Plectosporium, applying fungicides, and plowing under crop residue after harvest

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, 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, 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 preserve the useful life of the systemics is to make a single application from each effective fungicide group (anilide, stobilurin and DMIs) in a given season beginning when powdery mildew is first detected. In addition, pathologists are now recommending that all systemic materials be applied with a contact fungicide to help slow resistance development.

Chlorothalonil (i.e. Bravo) is effective at controlling Plectosporium blight on pumpkins but does not work as well as many systemic fungicides on powdery mildew. It can be mixed with systemics, such as the DMIs, myclobutanil and triflumizole (Nova or Procure), or protectants like sulfur (i.e. Microthiol Dispress) which work well on powdery mildew but do not control Plectosporium blight or other important cucurbit diseases.

Contact fungicides such as copper hydroxide and maneb may aid in the control of Plectosporium blight and possibly other diseases, but they don’t have the efficacy to provide sufficient protection when used alone. So, fungicides must be mixed or alternated to produce a combination that will provide a full range of disease protection. Systemics must be alternated with fungicides outside of their group to prevent the buildup of resistance.

Recommended Pumpkin Spray Schedules for 2005

Scout pumpkin and summer squash plantings weekly for symptoms of both Plectosporium and powdery mildew. Examine the lower surface of 50 leaves for small (1/4″), white powdery mildew colonies and all plant parts for Plectosporium lesions. If powdery mildew is detected first, follow the first spray schedule (example 1). Pristine contains boscalid, a new effective systemic material for powdery mildew control, and pyraclostrobin, the same active ingredient found in Cabrio (Anilide + Strobilurin groups). Procure is a new DMI in the same fungicide group as Nova. Applications should be applied 7-10 days apart, and should be limited to a single application per season for each fungicide group. Many Connecticut growers used a 10-day schedule during the wet 2004 season, and had no problems with Plectosporium blight, powdery mildew, black rot (GSB) and scab.

All systemic sprays should be applied with a contact fungicide (e.g. chlorothalonil, copper or maneb). Sulfur (i.e. Microthiol Dispress) and chlorothalonil (i.e. Bravo) can be used for late-season sprays to rest the systemic materials and still provide effective control of Plectosporium blight, powdery mildew, black rot (GSB) and scab. If downy mildew is found in Connecticut prior to September, other systemics (i.e. Ridomil/Bravo) may be needed with sulfur late in the season. There is no need to control downy mildew on pumpkins during September, because this disease only affects leaves and not fruit.

If Plectosporium blight is detected before powdery mildew, apply chlorothalonil (i.e. Bravo) on a weekly basis until powdery mildew is found (see spray schedule example 2).

Caution: do not apply sulfur if temperatures exceed 90oF, before/with/after oil applications, or to melons due to phytotoxicity problems.

Spray schedule example 1:

  Spray #
   (PM detected) 1 2 3 4
Fungicides Pristine + copper Procure + Bravo sulfur + Bravo repeat if necessary (sulfur + Bravo)

In unusually wet weather, in unrotated fields, or if Plectosporium is detected before powdery mildew, start your spray program as soon as the disease is detected or at fruit set. Scout your fields weekly for symptoms of Plectosporium blight and powdery mildew. Apply chlorothalonil (i.e. Bravo) every 7-10 days until powdery mildew is found during weekly scouting trips. Then add a systemic material or sulfur to the spray mix for mildew control, taking care to alternate between fungicide groups to help prevent resistance. Copper or maneb can be used with the Pristine application to rest Bravo. Crop rotation is an essential component of this management program.

Spray schedule example 2:

  Spray #
  (fruit set) 1-3? (PM detected) 4? 5? 6? 7? 
Fungicides Bravo Pristine + copper Procure + Bravo sulfur + Bravo repeat if necessary
(sulfur + Bravo)

(Reminder: Thorough coverage of the foliage and fruit with 40+ gallons of water per acre is recommended).

By: T. Jude Boucher, University of Connecticut, Cooperative Extension System. Reviewed 2012.

Special thanks to the following plant pathologists for their comments and suggestions in preparing these recommendations: R. Wick, S. Douglas, M. McGrath, T. Zitter, M. Babadoost, and K. Everts.

Information on our site was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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