Facts in Depth

What Is It? | Facts in Depth | For the Professional Diagnostician
Cucurbit Diseases | Cucurbit Downy Mildew

Cucurbit Downy Mildew

Identification

Cucurbit downy mildew symptoms occur only on the foliage of affected plants. Initial symptoms are small, chlorotic spots on leaves. Affected leaves exhibit chlorotic, yellow-olive, often angular, vein-delimited lesions that become tan and necrotic as the disease progresses.  Initial lesions are usually scattered across the leaf, but as disease progresses, larger areas become affected and lesions coalesce.  Fuzzy, white to dark gray-purple sporulation occurs on the undersides of lesions.  Severe infections can kill the leaf completely.

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Pathogen Biology

P. cubensis is an obligate pathogen, meaning it only survives on living host tissue. Cucurbit downy mildew epidemics are initiated by airborne sporangia blown in from areas in which cucurbits are produced year-round.  The sporangia germinate into zoospores and zoospores enter the plant through stomata.  Sporangiophores develop through the stomata of infected plants and release sporangia. Sporangia can be spread by water splashing, rain, or wind.  Under disease conducive conditions, as little as four days can pass between initial infection and inoculum production.  P. cubensis is capable of forming oospores, although the role of oospores in survival and infection is not completely understood for this pathogen.  Six pathotypes of P. cubensis have been described based on their ability to infect different cucurbits.  The chart below shows the host range of each pathotype (filled boxes indicate disease; pathotype 6 has only been reported in Israel and different pathotypes are likely present in Europe).

downymildewracetable

Favorable Environmental Conditions

P. cubensis requires high humidity (100%) for infection and spread to occur. Long leaf wetness periods are conducive to disease and can result from dews or rains. P. cubensis can cause disease under a wide temperature range, but the ideal temperature range for reproduction is 59-68° F. 

Often Confused With

  • Angular leaf spot: Check for bacterial streaming.  No streaming will be present if it is downy mildew.
  • Cercospora leaf spot: Place samples in a moisture chamber and look at what type of sporulation is present microscopically.
  • Powdery Mildew: These diseases can co-occur and chlorosis from powdery mildew can be confused with downy mildew.  Check sporulating areas under the microscope.

Scouting Notes

Scouting should occur on a regular basis as this disease can spread rapidly.  Areas of the plant that have heightened humidity, such as the undersides of leaves, should be scouted for the disease.  Areas of fields that have higher humidity (shaded areas, weedy areas) should be scouted for the disease.

The Cucurbit Downy Mildew Integrated Pest Management Pest Information Platform for Extension and Education (Cucurbit ipmPIPE) can be used to track and predict cucurbit downy mildew outbreaks and track which host plants have been reported in a given area.

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Thresholds

This disease spreads rapidly under disease conducive conditions.  Use of prediction (Cucurbit ipmPIPE) to time fungicide sprays can help maintain disease under grower set thresholds.

Management

  • Use resistant cultivars – Resistance cultivars are available for some types of cucurbits, including cucumber and melon.
  • Use sound cultural practices – Cultural practices to reduce humidity within the plant canopy can help limit disease. Space plants to allow for airflow and ensure weeds are controlled to reduce humidity. Use drip irrigation whenever possible.  Do not irrigate at night or extremely early in the morning to reduce humidity on the leaves.
  • Use fungicides – Fungicides can be used to manage this disease.  See the Midwest Vegetable Growers Guide for the latest fungicide recommendations.