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Phytophthora Blight

This disease has caused substantial losses (including total crop loss) on many farms on Long Island, and elsewhere in the Northeast, since around 1990.  Losses are due to fruit being affected and also plant death.  Cucurbit crops, in particular pumpkins and winter squash, and peppers have been affected most. Eggplant, tomato, and snap beans have also been affected.

Research was started in 1992 and has included evaluating fungicides and a large diversity of cultural management practices partly recognizing that soil-borne pathogens can be difficult to manage with fungicides alone.

Losses have been less in recent years at least partly due to registration of several targeted fungicides, and also implementation of cultural practices, as well as generally drier summer conditions.  Additionally, with repeat occurrences of Phytophthora blight in a field there has been indication of decreased disease incidence which could be due to an increase in antagonistic microbes.  However, with climate change there has been increasing occurrence of intense rainstorms creating saturated soil which is favorable for onset of Phytophthora blight when soil was moist beforehand.

Research Topics, Results and Reports

 

Fungicide Evaluations – Conventional Products

Most experiments were conducted in a field dedicated to research on this disease.  Squash, pumpkin, and pepper were used.  Fungicides were applied weekly on a preventive schedule (starting before symptoms seen) using a tractor-sprayer or a back-pack sprayer.  Percent fruit with symptoms was the main rating used to assess treatment efficacy.  Plant mortality was assessed in some experiments.  Disease incidence varied substantially among years partly reflecting occurrence of large rainfalls creating favorable conditions.  Inoculum, typically infected fruit, was put in the experiments when no symptoms were seen by late summer (see reports).  Overhead irrigation was used when needed for inoculum spread and to create favorable conditions, which was not as effective as expected.  Large variation among plots in some experiments resulted in lack of statistical significance among treatments although there were large numerical differences.  Variation was not consistent year to year making it difficult to effectively use an experimental block design.

Phytophthora blight was effectively controlled with fungicides in some experiments, but not others (numerically less disease but not statistically significant), documenting that this is a difficult disease to manage with fungicides alone, especially for controlling fruit rot in pumpkin as this symptom often starts on the fruit underside.  Several cultural management practices are recommended.  Additionally, presence of nontreated control plots creates higher disease pressure in experiments than there would be in a commercial planting treated with fungicides.

In 2022, a fungicide program with an alternation of Presidio, Omega, Orondis Ultra, Ranman, and Revus applied with a tractor-sprayer controlled fruit rot: there were 63% vs 88% fruit affected in nontreated plots on 12 October.

In 2007, all 5 fungicide treatments effectively controlled Phytophthora fruit rot in acorn squash: 9-22% fruit rotted by Nov 1 versus 71% in non-treated plots. Treatments were applied weekly with a back-pack sprayer starting 3 weeks after seeding. They were programs consisting of targeted fungicides applied in alternation (Forum, Presidio, Ranman, Revus, Revus Opti, ProPhyt, Tanos) and combined with contact fungicides (copper, maneb, and/or chlorothalonil).  There were 2 treatments with only copper or chlorothalonil applied on weeks when rain was not forecast.

In 2006, fungicides were applied with a back-pack sprayer to a susceptible pepper variety (Red Knight) and a resistant variety (Aristotle) in adjacent experiments.  Less disease developed on Aristotle: 55% of nontreated plants had died by 2 October vs 89% for Red Knight.  All fungicides tested (Presidio [tested as experimental V-10161], Ranman, Revus, Ridomil Gold Copper, and 2 experimentals not developed) were effective: 4-18% of treated Aristotle plants and 11-47% of Red Knight plants were dead on 2 October.

Three experiments were conducted in 2005 with summer squash and pepper.  Conditions were dry and thus unfavorable for Phytophthora blight during most of the growing season. Symptoms of Phytophthora blight were first observed on 7 and 30 September on squash and pepper, respectively.  Experiment 1: fungicides were applied with a tractor-sprayer to summer squash. Although incidence of fruit rot and crown rot was greater for the nontreated plants (with 1 exception), and usually substantially greater, differences were only statistically significant for crown rot incidence on 26 September with effective control (35-56% vs 95% incidence) obtained with Ranman + copper (Cuprofix Disperss), Revus (tested as experimental NOA 446510) + copper, Presidio (experimental V-10161) + Previcur Flex + copper, and Tanos + copper. Fruit rot incidence on 19 September was 10-54% for fungicide treatments and 66% for nontreated.  Other fungicides tested were Gavel + copper, ProPhyt + copper, Forum + copper, Forum + ProPhyt, and Reason + Previcur Flex + copper.  In the other 2 experiments, fungicides were applied with a back-pack sprayer to pepper and summer squash.  None of the treatments were effective.  They were programs with alternations and tank-mixtures of Forum, Gavel, Tanos, Kocide 2000, Manex (discontinued), and Manzate plus 2 experimentals: DPX-JE874 (AI= famoxadone) and DPX-GFJ52 (copper).

In 2003, none of the treatments applied to summer squash with a tractor-sprayer were effective. Four days with rain in mid-September provided favorable conditions for disease development. Fungicides tested were Phostrol, Acrobat (renamed Forum) plus copper, Tanos (as experimental KP481) applied alone or combined with copper and alternated with Acrobat plus copper.

Bravo Ultrex was effective through mid-October in 1999 when applied to pumpkin with a tractor-sprayer.  Disease pressure was high starting in mid-September.

Acrobat MZ applied to pumpkin using a tractor-sprayer was effective for Phytophthora fruit rot in 1998 (but disease occurrence was low), in 1999 when there were 4 days in September with rainfall exceeding 1 in. and incidence of fruit rot was high (48% for Acrobat MZ vs 85% for nontreated on 5 October), and in 2000 (based on % healthy fruit; disease occurrence was again low despite frequent often excessive irrigation to create favorable conditions). Acrobat MZ is no longer available.  Dimethomorph, its active ingredient with targeted activity for oomycete pathogens, is in the fungicide that replaced it, Forum, which in 2023 is registered for this use.  However, Acrobat MZ applied to summer squash in 1998 using a tractor-sprayer was ineffective. Other fungicides also ineffective in this experiment were Aliette, Ridomil Gold, and dimethomorph plus copper. While not statistically significant, there were fewer affected plants and fruit with all fungicide treatments.

In 1997, Ridomil Gold was more effective than Acrobat (dimethomorph) when applied to summer squash with a backpack sprayer.

In 1994, Phytophthora blight started to develop and rapidly following 3.5 inches rain over 4 days.  None of the fungicides applied to pumpkin using a tractor-sprayer were effective, but all treatments had numerically fewer symptoms of fruit and crown rot than the nontreated control.  Fungicides tested alone or combined were Ridomil 2E, Ridomil Copper, Ridomil Bravo, Aliette, Kocide, Bravo C/M, Maneb (since discontinued), and fluazinam (active ingredient in Omega which in 2023 is registered for this use).  Incidence of affected pumpkin fruit was high for treated and nontreated fruit in 3 challenge inoculations used to test Aliette and Ridomil in a second experiment in 1994, which was done because of the observed high variation in disease occurrence among plots in field experiments.

In 1993, disease onset was late, with symptoms not found throughout the field until 28 September, but quickly became very severe.  65% of fruit were affected on 30 September for the nontreated control, increasing to 80% on 14 October.  None of the fungicides applied to pumpkin using a tractor-sprayer were effective, but all had numerically fewer affected fruit.  Fungicides tested alone or combined were Ridomil 2E, Ridomil Copper, Ridomil Bravo, Aliette, Kocide, Bravo C/M, and fluazinam (active ingredient in Omega which in 2023 is registered for this use).

In 1992, symptoms were first seen on 17 August near the end of an extensive period of rain with 5.8 inches falling on 7 days from 9 to 18 August.  Incidence of fruit rot quickly became high.   None of the fungicides applied to pumpkin using a tractor-sprayer were effective, but all had numerically fewer affected fruit.  Fungicides tested alone or combined were Ridomil 2E, Ridomil Copper, and Ridomil Bravo.

Publications (most reports linked here are also available at Plant Disease Management Report website):

  1. McGrath, M. T., Ghemawat, M. S., and Staniszewska, H. 1993. Evaluation of fungicides for managing Phytophthora crown rot and fruit rot in pumpkin under stringent conditions, 1992.  Fungicide and Nematicide Tests 48:172.
  2. McGrath, M. T., and Staniszewska, H. 1994. Evaluation of fungicides for managing Phytophthora crown rot and fruit rot in pumpkin, 1993.  Fungicide and Nematicide Tests 49:141.
  3. McGrath, M. T. 1995. Evaluation of fungicides for managing Phytophthora crown rot and fruit rot in pumpkin, 1994.  Fungicide and Nematicide Tests 50:145.
  4. McGrath, M. T. 1995. Evaluation of fungicides for managing Phytophthora fruit rot in pumpkin based on results from challenge inoculation, 1994.  Fungicide and Nematicide Tests 50:148.
  5. Shishkoff, N., and McGrath, M. T. 1998. Evaluation of fungicides for Phytophthora fruit rot and tip blight of summer squash, 1997.  Fungicide and Nematicide Tests 53:240.
  6. McGrath, M. T. 1999. Evaluation of Acrobat MZ for managing Phytophthora fruit rot of pumpkin, 1998. Fungicide and Nematicide Tests 54:232.
  7. Shishkoff, N., and McGrath, M. T. 1999. Evaluation of fungicides for Phytophthora fruit rot and tip blight of summer squash, 1998. Fungicide and Nematicide Tests 54:246.
  8. McGrath, M. T. 2000. Evaluation of Acrobat MZ for managing Phytophthora fruit rot of pumpkin, 1999.  Fungicide and Nematicide Tests 55:250.
  9. McGrath, M. T. 2000. Evaluation of plant growth enhancers plus a fungicide to manage Phytophthora fruit rot and powdery mildew of pumpkin, 1999.  Fungicide and Nematicide Tests 55:251.
  10. Shishkoff, N., and McGrath, M. T. 2000. Evaluation of fungicides for Phytophthora fruit rot and tip blight of summer squash, 1999. Fungicide and Nematicide Tests 55:264.
  11. McGrath, M. T. 2001. Evaluation of Acrobat MZ for managing Phytophthora fruit rot of pumpkin, 2000.  Fungicide and Nematicide Tests 2001:V79.
  12. McGrath, M. T. 2001. Evaluation of plant growth enhancers plus a fungicide to manage powdery mildew and Phytophthora fruit rot of pumpkin, 2000.  Fungicide and Nematicide Tests 2001:V78.
  13. McGrath, M. T. 2004. Evaluation of fungicides for managing Phytophthora blight of squash, 2003. Fungicide and Nematicide Tests 59:V054.
  14. McGrath, M. T., and Davey, J. F. 2006. Efficacy of DuPont fungicides for Phytophthora capsici in pepper, 2005. Fungicide and Nematicide Tests 61:V033.
  15. McGrath, M. T., and Davey, J. F. 2006. Efficacy of DuPont fungicides for Phytophthora capsici in summer squash, 2005. Fungicide and Nematicide Tests 61:V035.
  16. McGrath, M. T., and Davey, J. F. 2006. Efficacy of fungicides for Phytophthora capsici in summer squash, 2005. Fungicide and Nematicide Tests 61:V034
  17. McGrath, M. T., and Davey, J. F. 2007. Efficacy of fungicides for control of Phytophthora blight in pepper on crown rot tolerant and susceptible varieties, 2006. Plant Disease Management Reports 1:V131.
  18. McGrath, M. T., and Fox, G. M. 2008. Evaluation of fungicide programs for managing Phytophthora blight in cucurbits, 2007. Plant Disease Management Reports 2:V147.
  19. McGrath, M. T. and Downing, C. T. 2023. Efficacy of a Phytophthora blight fungicide program for powdery mildew and Phytophthora blight, 2022.  Plant Disease Management Reports 17:V063.

 

Plant Resistance Evaluations – Resistant Varieties

In 1998, the resistant variety Paladin was shown to be much less susceptible than Brigadier.  Stem canker was the main symptom seen.  No plants of Paladin died by 16 Oct while 96% of Brigadier plants were dead.  A few Paladin fruit developed symptoms. Disease onset was very late, with first symptoms not seen until 22 Sep, despite a) previous history of Phytophthora blight in cucurbit experiments in this field in 1992, 1995, and 1997, b) overhead irrigation on 31 Aug and 1 Sep to create favorable conditions, and c) having rows of yellow summer squash between the rows of peppers to serve as spreader rows.  Disease started to develop after affected summer squash fruit were placed in the spreader rows, followed by irrigating the field on 16 Sep.

In 2008, 4 resistant varieties (Aristotle, Declaration, Paladin, and Revolution) and an experimental variety from Syngenta Seeds (RPP 20809) were compared to 3 susceptible varieties in 2 parallel experiments.  Fungicides targeting Phytophthora blight were applied to all plants in 1 experiment on a preventive basis starting with a soil application. This enabled evaluating an integrated program (fungicides applied to a resistant variety) as well as fungicides applied to susceptible varieties.  Rain events in 2008 provided very favorable conditions for development of Phytophthora blight. Symptoms of crown rot were observed on a few plants of the susceptible variety Crusader on 26 Jun, 13 days after transplanting. RPP 20809 had the lowest incidence of plants with crown rot symptoms in both experiments, which was significantly lower than all other varieties in both experiments on 20 Oct except for Paladin and Aristotle in the fungicide-treated experiment. Aristotle had the numerically lowest total incidence of fruit affected by Phytophthora over all harvest dates (40% and 22% for the two experiments), but these means were not significantly different from the other varieties.  Non-fungicide treated RPP 20809 had a high percentage of affected fruit (71% versus 72-90% for the susceptible varieties), documenting that resistance being bred into peppers is to the foliar (crown rot) phase.  Best control of the foliar and fruit rot phases of Phytophthora blight was obtained with the integrated management program.

In 2012, 5 resistant varieties (Archimedes, Intruder, Paladin, Revolution, and Vanguard) and an experimental variety (PS-0994 1819) were compared to a susceptible variety in 2 parallel experiments.  Fungicides targeting Phytophthora blight were applied to all plants in 1 experiment on a preventive basis.  Phytophthora blight developed slowly in both pepper experiments for this study.  Symptoms were not seen in this experiment until 23 Aug whereas blight was observed on 30 Jul in pumpkin in another LIHREC research field, and on 26 Jun 2008 when a similar study was conducted in this research field. Symptom development was delayed in Revolution. This was the only variety with no symptoms on 23 Aug. While there were no significant differences among varieties in the No Fungicide experiment, the susceptible variety King Arthur had numerically more plants and more fruit affected than the resistant varieties with one exception. Percentage of plants and fruit with symptoms were numerically lower in the Fungicide experiment than the No Fungicide experiment, suggesting that the fungicide program was effective. In contrast with the No Fungicide experiment, King Arthur in the Fungicide experiment did not have more plants or fruit affected by Phytophthora blight than the resistant varieties, thus, in contrast with a similar study conducted in 2008, there was no evident benefit of using an integrated management program (fungicides applied to a resistant variety) over fungicides alone.

Publications (most reports linked here are also available at Plant Disease Management Report website):

  1. McGrath, M. T. 1999. Evaluation of a resistant pepper variety for managing Phytophthora blight, 1998. Biological and Cultural Tests 14:170.
  2. McGrath, M. T., and Fox, G. M. 2009. Evaluation of Phytophthora-resistant bell pepper cultivars with and without a fungicide program, 2008. Plant Disease Management Reports 3:V132.
  3. McGrath, M. T., Menasha, S. R., and LaMarsh, K. A. 2013. Evaluation of Phytophthora-resistant bell pepper cultivars with and without a fungicide program, 2012.  Plant Disease Management Reports 7:V016.

 

Plant Resistance Evaluations – Hard-Rinded Pumpkins

Pumpkin varieties that develop a hard rind (shell) as the fruit matures, similar to gourds, were shown to have a lower incidence of mature fruit with Phytophthora fruit rot than varieties with conventional, soft rind. Immature green fruit were susceptible, but conditions tend to be more favorable for Phytophthora starting in late summer. These experiments were conducted when the first hard-rind varieties (Apprentice, Lil’ Ironsides and Iron Man) became commercially available and there were several experimental varieties in development.

Publications (most reports linked here are also available at Plant Disease Management Report website):

  1. McGrath, M. T., and Davey, J. F. 2007. Hard-rinded pumpkin cultivar evaluation for Phytophthora fruit rot, 2006. Plant Disease Management Reports 1:V125.
  2. McGrath, M. T., and Fox, G. M. 2008. Hard-rind pumpkin cultivar evaluation for Phytophthora fruit rot, 2007. Plant Disease Management Reports 2:V144.
  3. McGrath, M. T., and Fox, G. M. 2009. Hard-rind pumpkin cultivar evaluation for Phytophthora fruit rot, 2008. Plant Disease Management Reports 3:V126.

 

Plant Resistance Evaluations – Search for Sources of Resistance

Publications (most reports linked here are also available at Plant Disease Management Report website):

  1. McGrath, M. T., Kyle, M. M., Superak, T. H., and Palmer, L. 1994. Sensitivity of Cucurbita breeding lines and varieties to Phytophthora crown rot and fruit rot, 1993.  Biological and Cultural Tests 9:26.
  2. McGrath, M. T., Superak, T.H., and Palmer, L. 1995. Sensitivity of Cucurbia pepo breeding lines and varieties to Phytophthora crown rot and fruit rot, 1994.  Biological and Cultural Tests 10:147.
  3. McGrath, M. T., and Superak, T.H. 1996. Sensitivity of Cucurbita pepo experimentals and varieties to Phytophthora crown rot and fruit rot, 1995.  Biological and Cultural Tests 11:116.
  4. McGrath, M. T., and Superak, T.H. 1997. Sensitivity of Cucurbita pepo experimentals and varieties to Phytophthora crown rot and fruit rot, 1996.  Biological and Cultural Tests 12:170.
  5. McGrath, M. T., and Superak, T. H. 1998. Susceptibility of pumpkin experimentals to Phytophthora fruit rot in pumpkin, 1997. Biological and Cultural Tests 13:175.
  6. McGrath, M. T., and Superak, T. H. 1999. Susceptibility of pumpkin varieties and experimentals to Phytophthora fruit rot in pumpkin, 1998. Biological and Cultural Tests 14:173.
  7. McGrath, M. T. and Sexton, Z. F. 2020. Evaluation of butternut squash experimental cultivars for resistance to Phytophthora blight and downy mildew, 2019.  Plant Disease Management Reports 14:V075.

 

Evaluations of Cultural and Alternative Management Practices

Publications (most reports linked here are also available at Plant Disease Management Report website):

  1. McGrath, M. T. 1995. Evaluation of yard-waste compost used along or in combination with fungicides for managing Phytophthora crown rot and fruit rot in pumpkins, 1994.  Biological and Cultural Tests 10:143.
  2. McGrath, M. T., and Senesac, A. 1995. Evaluation of dinitroaniline herbicides for managing Phytophthora in summer squash, 1994.  Fungicide and Nematicide Tests 50:159.
  3. McGrath, M. T. 1996. Two-year rotation used alone or combined with a fungicide program for managing Phytophthora fruit rot in pumpkin, 1995.  Fungicide and Nematicide Tests 51:142.
  4. McGrath, M. T., and Shishkoff, N. 1996. Evaluation of yard-waste compost used alone or in combination with fungicides for managing Phytophthora crown rot and fruit rot in pumpkin, 1995.  Biological and Cultural Tests 11:115.
  5. McGrath, M. T. 1997. Evaluation of solarization and sorghum sudan grass for managing Phytophthora crown rot and fruit rot in pumpkin, 1995-1996.  Biological and Cultural Tests 12:159.
  6. McGrath, M. T., and Shishkoff, N. 1998. Evaluation of oat straw mulch and ryegrass living mulch for managing Phytophthora crown rot and fruit rot in pumpkin, 1997. Biological and Cultural Tests 13:174.
  7. Shishkoff, N., and McGrath, M. T. 1998. Evaluation of cocoa shell, oat straw and black plastic mulch for managing Phytophthora fruit rot in pumpkin, 1997. Biological and Cultural Tests 13:173.
  8. McGrath, M. T. and Rangarajan, A. 2002. Evaluation of brewery-waste compost for managing Phytophthora blight of pumpkin, 2001.  Biological and Cultural Tests 17:V20.
  9. McGrath, M. T. 2005. Yard-waste compost amended to soil for managing Phytophthora blight of pumpkin, 2004. Biological and Cultural Tests 20:V010.
  10. McGrath, M. T. and Sexton, Z. F. 2018. Efficacy of Biofence for managing Phytophthora blight in cucurbits, 2017.  Plant Disease Management Reports 12:V070.

 

More information and photographs are at the extension website:
Phytophthora Blight and Its Management in Cucurbit Crops and Other Vegetables