Plectosporium Blight: A New Disease of Cucurbit Crops to Prepare For

This article of historical interest was written in June 2005 by Margaret Tuttle McGrath, Long Island Horticultural Research and Extension Center. Another news article on this topic Plectosporium Blight: A New Destructive Disease of Pumpkins and Summer Squash in New England is appended at the end of this article.

For updated information, see: Plectosporium blight on cucurbits.

Plectosporium blight (previously named Microdochium blight) is a new, potentially devastating disease.  Pumpkin, squash (especially zucchini) and gourd are susceptible. It was first observed in the USA in the late 1980s. It is believed to have occurred in MD in 1987. The first confirmed report was TN in 1988. It was first seen in New England (MA) in 1999.  Since 1988, Plectosporium blight has been reported in GA, AL, LA, NC, KY, VA, MD, DE, NJ, PA, IN, IL, OH, CT, MA, and most recently, NY. Several NY growers commented that they thought it had occurred on their farm after they saw photographs of symptoms during extension meetings in early 2004.  Symptoms of Plectosporium blight were confirmed on pumpkin and zucchini squash plants in eastern NY in Aug 2004 (http://www.apsnet.org/pd/searchnotes/2005/PD-89-0432A.asp) and in Oct 2004 on pumpkin fruit brought on to Long Island to restock pick-your-own fields.  In 2000, the first year Plectosporium blight was observed in IL, symptoms were found in all pumpkin fields examined and yield loss was as much as 50%. Severe losses occurred in 2003 in CT and MA, where the disease was confirmed on 7 farms and in several home gardens, probably because rainy weather provided favorable conditions.  Two of these farmers reported total loss of their squash (yellow and zucchini) and pumpkin crops.

Once observed in a state, Plectosporium blight usually has reappeared in following years, unless weather is unfavorable for disease development. It now occurs routinely throughout TN and VA.  Plectosporium blight can disappear after conditions become dry because the pathogen is highly dependent on rainfall.  Spores of this pathogen are dispersed by wind and rain.

This pathogen is thought to not be the same as the one occurring in Europe, but rather to be a mutant of this fungus (Plectosporium tabacinum) which is a common saprophyte often found on decaying vegetable debris.

Symptoms occur on leaf veins, stems, and fruit (Figures 1 and 2).  Lesions are white and have a very distinctive diamond to spindle shape that is characteristic for this disease.  They are small initially (<1/4 inch), but can expand and coalesce, causing the entire surface of stems, leaf veins or fruit handles to turn white.  Leaf tissue between veins is not affected, thus early symptoms are not as apparent as with other foliar diseases.  Leaves eventually die and collapse, often in a 10- to 25-ft diameter circle around the site of initial infection.  Dead vines can be so brittle they shatter when stepped on.  Spots are more circular on fruit, and they remain small and don’t coalesce.

(See images at the LIHREC Vegetable Pathology website.)

Management begins with rotation because the pathogen can survive in soil.  The minimum effective rotational period has not been determined; at least three years is recommended.  The pathogen is not thought to be seed-borne.

Genetic resistance has not been detected, but differences in susceptibility have been documented among pumpkin varieties.  An experiment conducted in TN in 2003 revealed that Sorcerer had fewer unmarketable fruit (50%) while Gold Strike had the most (85%). Other varieties in this experiment were Howden, Magic Lantern, Howdy Doody, Aladdin, Mother Lode, Autumn King, Gold Standard and Big Autumn.

Since disease development requires leaf wetness, irrigate with drip.  If that is not feasible, time overhead irrigation to minimize leaf wetness.  Scout fields after rainy weather.

Carefully inspect any pumpkin fruit brought onto your farm to re-stock pick-your-own fields.  Affected fruit typically do not breakdown and the spots could be mistaken to be oedema or another non-pathological blemish.

Fungicides presently available for use in NY are Dithane DF Rainshield, Bravo, Cabrio, Flint, and Amistar.  Since Plectosporium blight is a new disease in the US, it is not on the label of most fungicides registered for use on this crop group and demonstrated to be effective for Plectosporium blight.  The target pest as well as the crop must be specified on a pesticide label for the product to be legally used in NY.  Plectosporium blight is specified on the Dithane and Cabrio labels.  Unfortunately Dithane is not labeled for use on pumpkin, which is one of the more susceptible crops. NYS Department of Environmental Conservation granted FIFRA 2(ee) requests to add Plectosporium blight to the labels for Bravo, Amistar, and Flint in 2004.

Applicators need a copy of the FIFRA 2(ee) Recommendation in their possession when Bravo (EPA Reg. No. 50534-201-100), Amistar (EPA Reg. No. 100-1164), or Flint (EPA Reg. No. 264-777) are used for Plectosporium blight.  A copy (tif file) can be obtained on the web at http://pmep.cce.cornell.edu/pims/current/

• Click on ‘EPA Reg. Number’ under ‘Search for Products by’.
• Enter the registration number for one of the three fungicide names.
• Click on ‘details’ in the right column.
• Click on ‘NYS Labels/Docs’.
• Look under ‘Document Type’ for ‘2EE’.
• Click on the corresponding label /Document ID.

Bravo and Dithane are the primary fungicides recommended for Plectosporium blight.  These were as effective as the QoI (aka strobilurin) fungicides Cabrio, Flint and Quadris in an experiment conducted in GA in 2001 (see Table 1).  Bravo has also been effective in other university fungicide evaluations. Chlorothalonil fungicides have provided effective control in commercial production fields  as well (Figure 3). 

QoIs have only been recommended for Plectosporium blight when other labeled diseases (e.g. powdery mildew, downy mildew, gummy stem blight/black rot) were also occurring because of the high risk of selecting resistant strains of these or another pathogen that is also present.  Unfortunately, utility of QoIs for disease control in cucurbits has been reduced by development of resistant strains of the powdery mildew and black rot pathogens, which have been associated with control failure.  The QoI recommended in 2005 for powdery mildew and black rot is Pristine because it contains an additional active ingredient effective for these diseases (boscalid). Pristine is not registered to control Plectosporium blight per se, but it contains the same active ingredient (pyraclostrobin) that is in Cabrio, a product that is registered for control of the disease.  While Pristine has not been evaluated yet for Plectosporium blight, applying Pristine for powdery mildew may control Plectosporium blight based on the fact that the highest labeled rate for Pristine has the same amount of active ingredient (pyraclostrobin) as the lowest rate of Cabrio, which has been shown to be effective for this disease in university experiments and is labeled for this use.

A 7-day fungicide application interval is recommended during rainy weather.  Plectosporium blight was significantly more severe in pumpkins sprayed every 14 days than those sprayed weekly when conditions were favorable in an experiment conducted in MD.  Development of Plectosporium blight  is highly dependent on rainfall. Thus the interval can be extended during dry weather.  This disease has been observed to disappear under dry conditions.

In a rotated field, begin applying fungicides for Plectosporium blight when powdery mildew starts to develop.  This timing has worked well in fungicide efficacy experiments conducted in MD.  Start at flowering or fruit set where scouting is not done.  Start applications shortly after crop emergence when planting into a non-rotated field that had Plectosporium blight the previous year as recommended in TN.  A weekly spray program has been used in most fungicide efficacy experiments.

Plectosporium Blight: A New Destructive Disease of Pumpkins and Summer Squash in New England

T. Jude Boucher
University of Connecticut
Cooperative Extension System

Robert Wick
University of Massachusetts
Department of Microbiology

(Note: The original text was modified to make recommendations appropriate for New York State and to point out other concerns relative to the spray program. T. Zitter, Vegetable MD Online)

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)

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.