Vegetable Disease Research: Tools in the Battle Against Phytophthora

Over the past decade, Phytophthora blight has dramatically spread to “clean” farms. As a result, growers have seen healthy crops completely fail.
Vegetable Disease Research: Tools in the Battle Against Phytophthora - Articles

Updated: November 2, 2017

Vegetable Disease Research: Tools in the Battle Against Phytophthora

Asexual spore, Sporangia of Phytophthora capsici, host: Cucurbita spp. L. Photo: Paul Bachi, University of Kentucky Research and Education Center, Bugwood.org

The impact of Phytophthora capsici or Phytophthora blight continues to expand every year, and growers across the state tell me that they also have phytophthora on their farms. Phytophthora can last in the soil for seven or more years. It infects a wide range of vegetable crops but the crops most severely impacted have been pumpkins, winter squash, watermelon, and peppers.

Figure 1. Phytophthora capsici is an oomycete plant pathogen that causes blight and fruit rot of important commercial crops such as pumpkins, winter squash, watermelon, and peppers. Photo: John Esslinger, Penn State

Educational efforts have focused on keeping phytophthora off the farm and aimed at cultural practices that minimize the impact of an infection. In 2016 a new fungicide was used with success in pepper fields on a Luzerne County farm. This farm has a long history of battling phytophthora. The fungicide, Orondis, appeared to stop the spread of a phytophthora infection despite weather conditions that favored the disease. Research was conducted in 2017 to determine if the apparent benefits of Orondis could be repeated, if there are other control options that offer equal or better control, and if control can be obtained at a lower cost per acre. One of the treatments was an application of manure-based compost (Figure 2). The hope was that the biological activity stimulated by the compost would out-compete phytophthora in the soil. This control option could provide a viable phytophthora management option for organic vegetable growers.

Figure 2. Compost treatment. Photo: John Esslinger, Penn State

The Pennsylvania Vegetable Growers Association and the Pennsylvania Vegetable Marketing and Research Program provided a grant that made the research possible. A plot was planted on June 14, 2017, in Columbia County. The field in Benton, Pennsylvania was selected based on its history of phytophthora blight. Irrigation was available but due to adequate rainfall was not used until August 25, 2017.

Figure 3. Agriculture student Patrick McCarthy lays down black plastic mulch. Photo: John Esslinger, Penn State

A soil sample was taken, and recommended lime was applied on May 26, 2017. Black plastic mulch on 6ft. centers was laid on June 12, 2017 (Figure 3). Three randomized replications of each of the four treatments were established. The manure compost was worked into the soil in the compost treatment replications on May 22, 2017. The plot was planted on June 14, 2017. The peppers were planted in a double row 25’ long. One of the double rows was planted to the cultivar Zsa Zsa and the other row was planted to the cultivar Playmaker. Zsa Zsa was selected because of its susceptibility to phytophthora and Playmaker was selected because of its resistance to phytophthora.

Treatments

The four replicated treatments were (1) Orondis, (2) Revus Nu-Cop HB rotated with Tanos Nu-Cop HB, (3) manure compost, and (4) untreated check.

Treatment 1:

Orondis Gold was applied one time only as a drench at the rate of 2.4 oz./acre on June 28, 2017. The label on the Orondis used in the research recommended is 2.4 oz. to 9.6 oz. per acre. Current Orondis labels recommend 4.8 oz. to 9.6 oz. per acre.

Treatment 2:

The Revus and Nu-Cop rotated with Tanos and Nu-Cop treatments were applied to the treatment 2 areas on a weekly rotating basis starting July 27 through September 12, 2017. Revus and Nu-Cop were applied in alternating weeks with the Tanos and Nu-Cop. Tanos was applied at 8 oz. /acre and Revus was applied at 8 oz. /acre (label recommendation) and the Nu-Cop was applied at 1 lb./acre. The Revus and Nu-Cop tank mix was applied on August 4, August 17, and September 1.

Treatment 3:

The manure (dairy) compost was applied broadcast at the rate of 3 tons/acre on May 22, 2017. The compost was shallowly worked into the soil immediately after application.

Treatment 4:

The check did not receive any fungicide or soil treatments.

Figure 4. First infection on Zsa Zsa. Photo: John Esslinger, Penn State

The plot received 1.5 tons of high calcium lime based on soil test recommendations. The plot was fertilized with 80-80-80 on June 1, 2017. The plastic mulch was laid on June 11, 2017. Phytophthora was not observed in the plot from the time of planting through the middle of August, so the plot was irrigated on August 25th and inoculated with phytophthora on August 28th. Phytophthora was first observed on Zsa Zsa fruit in the untreated check on September 15, 2017 (Figure 4). On September 21, 2017 each plant was evaluated for the presence of phytophthora (Figure 5).

Figure 5. Photo: John Esslinger, Penn State

Results

The table below indicates the number of plants that had at least one fruit with phytophthora symptoms out of the 75 plants that made up each treatment.

Treatments:Zsa ZsaPlaymaker
Orondis13 b22 b
Tanos & Copper, Revus & Copper2 a1 a
Manure compost35 c25 b
Untreated check24 c38 c

The Tanos Copper, Revus Copper treatment out-performed the other treatments. The Orondis treatment was second best. The manure compost had less disease than the untreated check with the Playmaker variety but not with the Zsa Zsa treatment.

Cost comparison of the treatments

  • Treatment 1 consisted of Orondis applied at 2.4 oz./acre which cost $65.00/acre.
  • Treatment 2 consisted of Tanos Nu-Cop (July 27, Aug. 8, Aug. 25 and Sept. 12) rotated with Revus Nu-Cop (Aug. 4, Aug. 17, Sept. 1) applied as tank mixes and sprayed over the top. Tanos was applied at the rate of 8.0 oz./acre, Revus was applied at 8.0 oz./acre, and Nu-Cop was applied at 1 lb./acre.
    1. Tanos cost per/acre $19.33 X 4 applications = $77.32/acre
    2. Revus cost per/acre $21.64 X 3 applications = $64.92/acre
    3. Copper cost per/acre $6.00 X 7 applications = $42.00/acre
    4. Total cost of treatment 2 per acre was $184.24. (does not include the cost of 7 applications)
  • Treatment 3 consisted of 3 tons of manure compost/acre which cost approximately $180.00/acre.
  • Treatment 4 had no additional costs.

Conclusions

  • Treatment 1 did a good job but there was enough diseased fruit to cause concern. Since Orondis was only applied once and at the lowest labeled rate the cost per acre was significantly lower than the other treatments. Syngenta has increased the lowest labeled rate from 2.4 oz./acre to 4.8 oz./acre. The 4.8 ounce rate gives better season-long control and would still be the lowest cost treatment.
  • Treatment 2 performed very well. While the treatment did not totally prevent disease development, it did keep disease to a manageable level. The cost was relatively high.
  • The treatment 3 compost had little or no benefit in managing the disease. The treatment cost was high compared to the fungicidal benefit.

There was not a significant difference in susceptibility of the Zsa Zsa and the Playmaker. This may be explained by the fact that since the Playmaker is a large fruited bell pepper. Due to the weight of the fruit (no harvesting was done) the plants tended to lean over allowing the fruit to come in contact with the soil (Figure 6). The Zsa Zsa is a smaller fruited pepper that tended to hold the fruit up off the soil.

Figure 6. Photo: John Esslinger, Penn State

Authors

Vegetables Fruit Greenhouse

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