Arkansas Fruit, Vegetable and Nut Update

Disease Management in Pumpkins

by Aaron Cato - September 9, 2022

Pumpkins are a great crop to grow in the Midsouth to diversify farms and support agritourism. Pumpkins are typically thought of as low maintenance compared to many other specialty crops like tomato or strawberry, however, pumpkins do require weekly scouting for both diseases and insect pests, along with a preventative fungicide spray program. In the Midsouth region, a basic fungicide program is necessary for disease prevention, and more targeted fungicides should be used if some major pumpkin diseases are present. See below for information about what types of tactics to integrate into sustainable management plans, and how to scout for major diseases that may need additional fungicides. Please consult the Southeastern US Vegetable Crop Handbook for additional recommendations.

Cultural Control Tactics

Much of the disease risk in pumpkins can be mitigated by integrating cultural control tactics, sanitation and the use of resistant varieties into your production program. First, plant pumpkins in well-drained areas. We’ve seen many cases of soil-borne pathogens taking over fields that don’t drain well or are quickly inundated with water. Next, plant earlier when possible. Planting windows are generally based on market sources but being on the front part of that window can avoid increased risk of powdery and downy mildew that comes in the late Summer/early Fall.

Third, remove all dying plants as they are found, especially before transplanting into the field. Next, avoid overhead irrigation. Many diseases are a function of leaf-wetness and sprinkler irrigation can lead to increased risk. Drip irrigation is a great option for both production efficiency and disease prevention. Last, utilize crop rotation where possible. Integration of these tools is an important step to mitigate risk from many of the diseases that will be mentioned below.

Fungicide Spray Program

Pumpkins can be infected by a wide variety of pathogens and exhibit many diseases. Many of these diseases are prevented by implementing a very basic spray schedule. Pumpkins should receive an application of a fungicide that contains either chlorothalonil or mancozeb every 10 to 14 days. This application interval should be tightened to 10 days based on rainfall events that create a more ideal environment for disease-causing pathogens and are likely washing off protectant fungicides. This basic spray program along with cultural control tactics will likely get many growers through the entire growing season.

There are three diseases that all pumpkin growers need to scout for weekly to prevent major losses. These diseases are downy mildew, powdery mildew, and plectosporium blight (white blight). Each of these diseases occurs to varying degrees in Arkansas every year, and often growers can tell you which they tend to deal with. The good thing is that growers can respond to these diseases when they occur and don’t typically need to use anything other than our basic spray schedule for prevention prior to these diseases occurring in their pumpkins.

Below is a decision tool that growers can use to choose their foliar fungicides throughout the year (Figure 1). It should be noted that the basic spray program will help to prevent these diseases throughout the season. Each disease will be discussed at length below to provide a guide on how to identify what’s damaging your pumpkins. As always, many of these diseases can be difficult to identify based on leaf symptomology, and I recommend sending a sample into the Plant Health Clinic for confirmation.

Figure 1. Decision tool for pumpkin fungicide spray schedule. Chlorothalonil or mancozeb should be utilized every 10-14 days even if no disease is found to be present. Add additional fungicides if powdery mildew, downy mildew, or white blight is observed.

Figure 1. Decision tool for pumpkin fungicide spray schedule. Chlorothalonil or mancozeb should be utilized every 10-14 days even if no disease is found to be present. Add additional fungicides if powdery mildew, downy mildew, or white blight is observed.

Powdery Mildew

Cucurbit powdery mildew is a common disease in Midsouth pumpkin production that can be easily managed. Cucurbit powdery mildew is caused by the fungal pathogens Podosphaera xanthii and Erysiphe cichoracearum. This disease causes white powdery growths on leaves, stems, and petioles (Picture 1), which then leads to necrosis as the disease progresses. Infection generally starts in the shaded areas of plants on older leaves, but this diseases can quickly cover entire plants. Powdery mildew leads to an overall decline in plant health and lower plant yields, and can quickly colonize large portions of fields and lead to great economic loss.

Powdery mildew should be scouted for each week and is generally more severe after fruit set and in more dense plantings. Growers should scout the oldest leaves in the densest areas of their fields first, and should also keep an eye out for spots in the field that are becoming chlorotic (turning yellow) (Picture 2). Powdery mildew should not be confused with genetic silvering present on many pumpkin cultivars (Picture 2). When powdery mildew begins to be found in fields, growers should add an effective fungicide to their spray program along with their base protectant fungicide on a 7-10 day spray interval. Fungicides effective against cucurbit powdery mildew include: Gatten (flutianil, FRAC U13), Luna Experience (fluopyram +tebuconazole), FRAC 7+3), Quintec (quinoxyfen, FRAC 13), and Vivando (metrafenone, FRAC 50). Growers should rotate effective modes of action between each spray to avoid potential issues with resistance, especially if utilizing quinoxyfen containing fungicides.

Picture 1. Cucurbit powdery mildew fungal spores present on the top of pumpkin leaves.

Picture 1. Cucurbit powdery mildew fungal spores present on the top of pumpkin leaves.

Picture 2. Cucurbit powdery Mildew spores, chlorotic symptomology, and genetic silvering on pumpkin leaves. Genetic silvering (A) can be confused with powdery mildew due to its color, but it will appear within leaf veins and have a blocky shape. Powdery mildew spores (B) look more like powder and damage will express as chlorosis (C) on the opposite side of the leaf.

Picture 2. Cucurbit powdery Mildew spores, chlorotic symptomology, and genetic silvering on pumpkin leaves. Genetic silvering (A) can be confused with powdery mildew due to its color, but it will appear within leaf veins and have a blocky shape. Powdery mildew spores (B) look more like powder and damage will express as chlorosis (C) on the opposite side of the leaf.

Downy Mildew

Downy mildew is one of the most devastating diseases of cucurbits. In Arkansas, downy mildew of cucurbits is only occasionally observed because the fungus that confers this disease, Pseudoperonospora cubensis, can only survive in living cucurbit plants. These plants do not survive the winter in Arkansas and therefore this disease does not overwinter in the state.

However, this disease does overwinter in states further South and it can work its way across the Southeast in the summer as spores are moved in the wind. This makes scouting very important for disease management, especially considering the limited number of products that provide good suppression.

Cucurbit downy mildew presents as angular, yellow/light-green spots early, which turn in to necrotic yellow/brown lesions on the tops of leaves (Picture 3). It can be further identified by gray/purplish spores on the underside of leaves (Picture 4). Growers should keep an eye out for these symptoms and check the forecasting tool that has been created to determine areas at the highest risk of infection. This tool works via reports from extension specialists of confirmed cases of downy mildew, which are then combined with weather data to determine the risk for surrounding areas. Please consult the CDM IPM Pipe Website for downy mildew forecasting to get out ahead of any potential issues. Provided below is an example of forecasting for downy mildew from 2020 (Picture 5). Current reports show no risk for the next few days, but expect this to change especially if a tropical storm system moves up through the gulf.

Growers should scout at least once a week for downy mildew in pumpkins. These scouting efforts should be intensified when forecasts indicate a risk for our area, or when we have confirmed cases in the state. Fungicide products that provide good control of downy mildew are Elumin (ethaboxam, FRAC 22), Orondis Ultra (oxathiapoprolin FRAC 49 + mandipropamid FRAC 40), Orondis Opti (oxathiapoprolin FRAC 49 + chlorothalonil FRAC M05), and Ranman (cyazofamid, FRAC 21). See the 2022 Southeastern US Vegetable Crop Handbook for further fungicide recommendations and products that provide fair control/prevention. None of these products will provide good control as a salvage application, making it necessary to scout often and watch the IPM Pipe forecasting.

Picture 3. Downy mildew symptomology observed in Clay county. Leaves toward the bottom of the picture show signs of early symptomology with yellow angular lesions. Many leaves of this picture show advanced symptomology with leaves rolling inward. Photo by Allison Howell, Clay County UAEX.

Picture 3. Downy mildew symptomology observed in Clay county. Leaves toward the bottom of the picture show signs of early symptomology with yellow angular lesions. Many leaves of this picture show advanced symptomology with leaves rolling inward. Photo by Allison Howell, Clay County UAEX.

Picture 4. Angular lesions exhibiting grey/purple spore growth on the underside of the leaf. This is a key characteristic of potential cucurbit downy mildew. Photo by Allison Howell, Clay County UAEX.

Picture 4. Angular lesions exhibiting grey/purple spore growth on the underside of the leaf. This is a key characteristic of potential cucurbit downy mildew. Photo by Allison Howell, Clay County UAEX.

Picture 5. Forecast for September 1st 2020 that indicated a high risk of cucurbit downy mildew for areas close to Arkansas. Current forecasts (September 1st, 2022) indicate no risk at this time, but this could change quickly.

Picture 5. Forecast map showing September 1^st 2020 that indicated a high risk of cucurbit downy mildew for areas close to Arkansas. Current forecasts (September 1^st, 2022) indicate no risk at this time, but this could change quickly.

Plectosporium Blight

Plectosporium blight, which is also referred to as “white blight,” is a common disease that can quickly overrun pumpkin plants and lead to quality and total fruit loss. Symptoms usually begin as white, diamond-shaped lesions that present on leaf midribs, stems, fruit handles, and the rind of fruit (Pictures 6 and 7). These lesions can eventually bleach sections of fruit or stems, and lead to decline in the plant or the fruit quality. Plectosporium blight is not usually a major issue in Arkansas, but it isn’t hard to find it sporadically in fields across the state. Growers should scout for this disease and try to discern whether problems are beginning to worsen.

Downward cupping of leaves and tan midribs can be a sign of worsening plectosporium blight, but abrasions should not be mistaken for lesions. A basic fungicide spray program along with cultural controls should prevent this disease in most cases, especially since it is more commonly observed in wet conditions. If symptoms begin to rapidly progress and if excessive rainfall persists, use one of the following fungicides on a 7-10 schedule along with the basic spray schedule: Cabrio (pyraclostrobin, FRAC 11) or Flint (trifloxystrobin, FRAC 11).

Picture 6. Plectosporium blight lesions present on pumpkin stems. Lesions have progressed and are beginning to bleach portions of this plant. Effected areas of the plant are more brittle and prone to breaking.

Picture 6. Plectosporium blight lesions present on pumpkin stems. Lesions have progressed and are beginning to bleach portions of this plant. Effected areas of the plant are more brittle and prone to breaking.

Picture 7. Plectosporium blight lesions present on a ripe pumpkin fruit. Photo: William Nesmith, University of Kentucky.

Picture 7. Plectosporium blight lesions present on a ripe pumpkin fruit. Photo: William Nesmith, University of Kentucky.