MSU Extension

Bacterial diseases were present in pumpkin fields in the 2014 growing season. In Michigan, the most common bacterial disease in pumpkins is angular leaf spot. However, bacterial leaf spot, a different disease, is emerging in the Midwestern United States. Understanding the similarities and differences between angular leaf spot and bacterial leaf spot can help you manage them better and prepare for the 2015 growing season starting this fall.

Angular leaf spot (ALS)

Most growers are familiar with the symptoms of angular leaf spot caused by Pseudomonas syringae pv. lachrymans. Characteristic symptoms of ALS in cucurbit foliage (cucumber, melons, squash and pumpkin) are irregularly-shaped lesions that are water soaked when young, and bleach to gray as lesions expand. The center of the lesion becomes brittle and breaks, leaving a “shot hole” on the leaf surface. Early lesions on the fruit are water soaked and oval to circular (0.04-0.2 inches) in shape. Under humid conditions, bacterial exudates can ooze from the lesions in leaves and fruits and when dry the exudate looks like a whitish residue.

Angular leaf spot symptoms in pumpkin leaf. Photo by Lina Rodriguez Salamanca, MSU Extension

Pseudomonas syringae pv. lachrymans is seedborne, specifically associated with the seed coat, therefore infection occurs as early as early as cotyledons emerge. This pathogen is dispersed by rain. Insects, machinery, labor clothing and hands can also aid plant-to-plant dispersal.

This bacterial pathogen can be found in irrigation water. Overhead irrigation represents not only a high risk for the potential introduction of the pathogen in the fields, but also can create extended humidity periods conducive to disease development. The bacterium can survive winter associated with plant debris.

Bacterial leaf spot (BLS)

Bacterial leaf spot is caused by Xanthomonas cucurbitae (syn=X. campestris pv. cucurbitae). Lesions appear first on the underside of the leaves as small, water soaked dots that look yellow from the upper side of the leaf. Lesions are especially small (0.07 inches) in pumpkin, winter squash and gourd leaves. As lesions enlarge (0.07-0.15 inches), they can coalesce and look like ALS. Fruit lesions start as sunken, circular spots (0.04-0.1 inches) that enlarge and can reach up to 0.6 inches in diameter. These openings allow the colonization of the fruit by saprobes or secondary microorganisms that can cause fruit rot in the field or post-harvest.

Ongoing field research by Mohammad Babadoost, University of Illinois at Urban-Champaign, and collaborators has found that X. cucurbitae can survive in the soil for 24 months when associated with plant tissue.

Pumpkin leaves infected with Xanthomonas cucurbitae. A, a leaf with translucent lesions; B, a leaf with angular spots; C, a leaf with dark lesions with yellow margins; D, a severely infected old leaf. Photo by Mohammad Babadoost, University of Illinois at Urbana-Champaign

Xanthomonas cucurbitae is a seedborne pathogen, favored by high precipitation or overhead irrigation. Before 2005, outbreaks were considered sporadic in the Midwestern United States. According to recent research by Ravanlou, et. al, the occurrence of this disease in the Midwest has increased over the last eight years. The disease was observed in more than 85 percent of the fields sampled across eight states, including Illinois, Indiana, Iowa, Kansas, Missouri, Nebraska, Ohio, Wisconsin and Michigan, causing as much as 90 percent yield loss in severely affected fields. In the last three years, bacterial leaf spot outbreaks have had a significant impact on the pumpkin industry in Illinois. The Michigan fields sampled during 2011 and 2012 (five fields each year) by Ravanlou and collaborators had a lower percentage of bacterial leaf spot-infected fruit per field when compared to other Midwestern states sampled.

Bacterial leaf spot symptoms in pumpkin fruit. Photo by Mohammad Babadoost, University of Illinois at Urbana-Champaign

What does ALS and BLS management have in common?

Since both bacterial pathogens are seedborne, disease management starts with the use of pathogen-free seed. Seed treatments (dry heat, hot water, sodium hypochlorite, etc.) can reduce the bacterial numbers in the seed, but will not eliminate it completely.

When ALS or BLS have been confirmed in a specific field, rotate away from cucurbits for two years or longer. Avoid working fields when plants are wet (morning dew or after rain) as this minimizes bacterial spread from diseased to healthy plants. If irrigation is needed, avoid using overhead irrigation to minimize bacterial pathogen dispersal. Avoid using surface water for irrigation as several species of Pseudomonas and Xanthomonas have been documented in surface water.

Frequent foliar application of preventative sprays can help decrease the bacterial population in the field to some extent.

Preventative sprays considerations

The efficacy of copper products and other materials to manage ALS and BLS is limited. Preventive application of copper can reduce the number or plants infected and the severity of disease development in the field, but has limited efficacy on years with high rainfall. Preventative application of copper formulations is considered more efficacious compared with sprays after the symptoms have developed.

Different copper formulations are available. However, it is important to tank-mix or alternate with other products such EBDC, Manzate, Actigard, Tanos or Serenade. Michigan State University Extension reminds growers to always read the labels. The development of copper resistance is a growing concern; alternating different modes of action is a tool to prevent and manage copper resistance.

Keep in mind that copper formulations should not be applied in solutions having a pH below 6.5. As pH decreases, more copper ions become available and can cause damage in leaves or fruits. However, the efficacy of copper formulations can be impacted at basic pH. A pH range of 6.5 to 8 allows for available copper ions while decreasing the risk of phytotoxicity.

For more information, see “Bacterial Spot of Cucurbits” from University of Illinois Extension.

For more information on Midwest survey and treatments tested to manage BLS:

• “Occurrence of bacterial spot (Xanthomonas cucurbitae) in cucurbit fields in the Midwestern of the United states” by Ravanlou, A., Liu, Q., Thapa, S., Zhang, X., and Babadoost, M.
• “Outbreak of Bacterial Spot (Xanthomonas cucurbitae) in Pumpkin Fields in Illinois” by M. Babadoost and A. Ravanlou

For more information on seed treatments:

• “Hot Water and Chlorine Treatment of Vegetable Seeds to Eradicate Bacterial Plant Pathogens” by Ohio State University
• “Evaluation of the efficacy of chemical and physical seed treatments in reducing the incidence of bacterial spot on pumpkin seedlings” by X. Xu, S.A. Miller and M.L. Lewis Ivey