Plectosporium blight caused by the fungus Plectosporium tabacinum (formerly known as Fusarium tabacinum and then Microdochium tabacinum) has arrived in southern New England. This disease was first reported in the U.S. in Tennessee in 1988 and in Virginia in 1994. It has since occurred on an annual basis in Virginia. Plectosporium blight destroyed half the pumpkin crop in Illinois in 2000. It has now been reported in most states east of the Mississippi River, but has not yet been documented in New York or northern New England.

There was a single occurrence on a Massachusetts farm in 1999, which has not had a problem with the disease since, perhaps owing to crop rotation. However, pathologists confirmed the occurrence of the disease on four Connecticut and three Massachusetts farms in 2003, as well as in several backyard plantings. Based on grower descriptions of cucurbit crop problems, it is believed that P. tabacinum was far more wide spread than indicated by these few confirmed cases. The 2003 season was one of the wettest years on record in southern New England and many growers may have dismissed crop loses as normal fruit rot problems associated with wet weather.

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 and zucchini. One large Connecticut farm lost 60-70% of their pumpkin crop in 2003. Another small farm lost all its pumpkins. A third farm lost all its zucchini and summer squash plants in a 2 week period in late July and early August, while the last Connecticut farm experienced only mild vine and foliar symptoms on summer squash.

A Case Study of an Epidemic on One Connecticut Farm

A large pumpkin producer, direct seeded"Howden" pumpkins in most of his fields between 8 and 11 June. Heavy, late-June rains delayed further planting, so that the remainder of his fields were transplanted on 1 July with a short-season jack-o-lantern variety and sugar pumpkins. His fields were located in three towns and most were separated by several miles.

The farmer followed the IPM fungicide program recommended in the state and used the same program on all his fields. Fungicide applications began on 30 July, just after powdery mildew was first detected on the underside of leaves during scouting. Three applications were applied 10 days apart. He alternated between the systemic fungicides azoxystrobin (Quadris) and myclobutanil (Nova) for resistance management and added chlorothalonil (i.e. Bravo) to the myclobutanil (Nova) application for better protection against a wider range of diseases (all fungicides are registered in NYS). The grower had a good crop rotation program and generally went 2 to 3 years before replanting pumpkins. Some of the 2003 pumpkin fields had been in field crops for 7 to 20 years.

Only the Howden pumpkin fields suffered damage from Plectosporium blight!  This left us with more questions than answers. Did the fungi arrive on Howden pumpkin seeds or were the other short-season varieties resistant?  If spores arrived on the wind, how had the disease skipped NY state?  Did somebody import infected fruit from another region?  Did the spores spread and infect pumpkin seedlings in late June, before the late season varieties were transplanted?  What type of spray program do we need in the future to prevent damage from Plectosporium blight and the rest of the cucurbit disease complex?  The good news is that this disease is easily recognized, if you know what you're looking for, and can be effectively managed if you know how. Hopefully, this fact sheet will help you recognize and effectively manage Plectosporium if it occurs on your farm this summer.

Description and Management                   

Plectosporium blight is favored by cool, humid or rainy weather. The fungus can overwinter on crop residue and can persist in the soil for several years. The fungus has not been reported to be seed-borne and no pumpkin or summer squash varieties are known to be resistant to the disease. Tiny, one or two-celled, sickle-shaped spores are formed in lesions on vines, stems, fruit, leaf veins and petioles. Spores are spread by rain-splash and wind. Lesions are small (<1/4 inch) and white. On vines, the lesions tend to be diamond to lens shaped; on fruit they are round to irregular (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. Fruit lesions produce a white russeting on the surface and stems that render the fruit unmarketable (fig. 5). The fruit lesions also allow for entry of soft rot pathogens that hasten the destruction of the crop (fig. 6). If you detect Plectosporium blight in your fields, please bring a sample in to the diagnostic laboratory for confirmation (Plant Disease Clinic, 329 Plant Sci. Bldg. Ithaca, NY)

Fig. 1. White, diamond-shaped lesions on vine.	Fig. 2. Lesions on leaf petiole.	Fig. 3. Round fruit lesions on pumpkin

 

Fig. 4. Severely infected vines become brittle.	Fig. 5. Lesions produce white russeting on fruit.	Fig. 6. Lesions allow soft rot pathogens in.

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 if necessary, and plowing under crop residue after harvest. Chlorothalonil (i.e. Bravo) and trifloxystrobin (Flint) are effective fungicides for controlling Plectosporium blight. 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 as well as most other important diseases of summer squash (Alternaria, gummy stem blight, scab, and downy mildew). However, fungicide selection is more challenging and expensive when it comes to choosing the most effective materials for chemical control on a long-season crop such as pumpkins.

(Reminder: Quadris and Amistar are extremely phototoxic to certain apple varieties)

One problem limiting the effectiveness of using chlorothalonil (i.e. Bravo) for control of Plectosporium blight on pumpkins is that it does not work as well as many systemic fungicides on powdery mildew. Although trifloxystrobin (Flint) works well to control Plectosporium blight and powdery mildew, it is not the best selection for some other important cucurbit diseases such as black rot (gummy stem blight). Other systemic strobilurin fungicides containing azoxystrobin (e.g. Quadris and Amistar) work pretty well on most of the important cucurbit diseases, but repeated use of systemics from the same fungicide group can hasten pesticide resistance and can lead to control failure. Powdery mildew and black rot have already developed resistance to stobilurins in some states (and resistance for downy mildew has occurred outside the US). The best resistance management strategy to help preserve the useful life of the systemics is to make a single application from each of the strobilurin (Group 11) and demethylation inhibitors (DMI) (Group 3) fungicide in a given season. New strobilurins containing pyraclostrobin (Cabrio) may be registered for use in New York in the near future, but it is still recommended that only a single application from this fungicide (Group 11) be used per season.

Other systemics, such as the DMI's myclobutanil and triflumizole (Nova or Procure) or protectants like sulfur (i.e. Microthiol Dispress), work well on powdery mildew but do not control Plectosporium blight or other important cucurbit diseases. Protectants such as copper hydroxide (i.e. Kocide, Champ, Cuprofix) and maneb may aid in the control of Plectosporium blight and other diseases (Bacterial leaf spot and Angular leaf spot), 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 build up of resistance. (These fungicides are listed as Group M, for multi-site activity)

Recommended Pumpkin Spray Schedules for 2004

If your farm did not have Plectosporium blight in the past, it is suggested that you scout pumpkin and summer squash plantings weekly for symptoms of both Plectosporium and powdery mildew. If powdery mildew is detected first, follow the first spray schedule (example 1). Strobilurin (e.g. Flint, Amistar or Quadris) and demethylation inhibitor (e.g. Nova or Procure) applications should be applied 7 to 10 days apart, and should be limited to a single application per season for each fungicide group. Use the shorter 7-day interval during periods of wet weather. All systemic sprays should be applied with a protectant 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 and other important cucurbit diseases. 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 90⁰F or with/after oil applications due to phytotoxicity problems.

Spray schedule example 1:

If your farm had Plectosporium blight last year, it is suggested that you start your spray program as soon as the disease is detected or at fruit set (especially, if it is a wet season). Scout your fields weekly for symptoms of Plectosporium blight and powdery mildew. Apply chlorothalonil (i.e. Bravo) every 7 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 strobilurin application to rest Bravo. Use a 10-day interval after a systemic and 7-day interval after Bravo. If the weather is wet, stay with a 7-day interval. Crop rotation is an essential component of this management program.

Spray schedule example 2:                                                            

(Reminder: Thorough coverage of the foliage and fruit is highly recommended, using 30-50 gallons of water per acre. )

Special thanks to Sharon Douglas, Plant Pathologist at the Connecticut Agricultural Experiment Station , Margaret Tuttle McGrath and Thomas Zitter, Plant Pathologists at Cornell University, for reviewing this article, and for helpful comments from Dr. M. Babadoost, University of Illinois.