Oklahoma gears up for Phytophthora blight fight

“The disease became readily apparent following the heavy rains of tropical storm Erin, although it first appeared before the storm in a low area of the field,” said John Damicone, Oklahoma Cooperative Extension Service plant pathologist.

Phytophthora blight is a plant disease that has become severe across much of the eastern United States in the past 10 years. It is highly dependent on moisture, and generally first develops in low areas of a field where water collects. It spreads with surface water movement, such as occurs with heavy rainfall.

On squash and peppers, it causes a crown rot that appears as an elongated, soft lesion that extends above the soil line, leading plants to wither and eventually die.

On watermelons and cucumbers, the main result is fruit rot. Fruit lesions become soft and covered with a white powdery growth that consists largely of asexual reproductive structures called sporangia.

“Sporangia can act like individual spores and germinate directly, or release numerous smaller spores called zoospores in the presence of water,” Damicone said. “Infested fields generally consist of two mating types that cross to form oospores.”

And that is where things can go from bad to worse. Oospores remain dormant in soil until another susceptible crop is planted, wherein oospores germinate to produce sporangia. Oospores survive indefinitely in the soil and reports exist of disease following crop rotations of 5 years to 10 years.

“Phytophthora blight is difficult to manage,” Damicone said. “The most effective control strategy is not to plant a susceptible crop in an infected field or in fields that receive drainage from problem fields.”

Producers should plant only in well-drained sites and avoid planting in low areas such as terrace bottoms: Good choices are raised beds or in fields that have been constructed to drain rapidly and completely.

“Alternately, use of no-till production systems following small grains helps promote water infiltration into the soil while the stubble minimizes splashing,” Damicone said. “Producers should time irrigation watering to minimize the creation of puddles.”

Sanitation is important. Producers should never dump culls – rotten fruit – from problem fields onto production fields.

“Phytophthora blight is a serious disease,” Damicone said. “Think of it as the plague: It’s that serious in terms of devastation and what it can do to a field.”

Additionally, when irrigating with surface water, producers need to ensure that the water source does not receive runoff from an infested field.

“That means the producer is also going to have to power-wash tractor tires and tillage implements after working problem fields,” Damicone said. “The best control is doing everything possible in terms of prevention.”

Results have been mixed following the use of fungicide programs to control Phytophthora blight.

“Mefenoxam/metalaxyl has been applied to pre-plant incorporated and as a foliar spray in combination with copper hydroxide,” Damicone said. “However, strains resistant to this fungicide have developed, rendering it ineffective in many areas.”

Intensive foliar spray programs have relied mostly on Gavel®, zoxium plus mancozeb; Acrobat®, dimethamorph; and Tanos®, cymoxanil plus famoxadone; applied alone or in combination with copper hydroxide.

Damicone said positive effects have been reported, but the degree of disease control is often low – 50 percent or less – and inconsistent among trials conducted in various states.

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REPORTER/MEDIA CONTACT:
Donald Stotts
News and Media Relations Manager
Agricultural Communications Services
143 Agriculture North
Oklahoma State University
Stillwater, OK 74078
Phone: 405-744-4079
Fax: 405-744-5739
E-Mail: donald.stotts@okstate.edu

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