Summer brings the possibility of Phytophthora rot

Andrew Kness
Andrew Kness
Extension Educator, Agriculture
University of Maryland Extension, Harford County

As summer rolls along, one disease issue that can affect a wide range of vegetable crops is Phytophthora rot. Staying ahead of this yield-robbing disease can help save you money.

Phytophthora rot is caused by the plant pathogen, Phytophthora capsici. This pathogen is an oomycete, morphologically similar to the true fungi, but is more closely related to algae than fungi. Like fungi, oomycetes have vegetative tissue called hyphae which forms a mat of mycelium (the white, thread-like stuff you see on a diseased plant). However, the hyphae of oomycetes are made up of cellulose, whereas the cell walls of the true fungi are made up of chitin. This is an important distinction that plays a big part in chemical management of this disease, as we’ll see later.

P. capsici can cause disease on over 50 plant species, many of which are high-value fresh market and processing vegetables. These include pepper, tomato, eggplant, lima bean, snap bean, and all members of the cucurbit family, as well as many common weed species.

In the presence of opposite mating types, P. capsici can produce thick-walled, overwintering spores called oospores. Oospores can survive in the soil without a host for at least 5 years [1]. Asexual overwintering spores, infected crop debris and weeds are also possible sources of primary inoculum. Extended periods of leaf wetness (via heavy and prolonged rainfall, over-irrigation, or flooding), high humidity, and warm temperatures (75-90 F) are required for spore germination. Infection usually occurs on fruit deep in the leaf canopy and/or fruit in contact with the soil. P. capsici may infect stems and leaves of some species of plants. Once primary infection occurs, sporulation can occur in as little as 2-3 days under optimal conditions [2]. Sporulation of asexual fruiting bodies, called sporangia, typically appear on infected fruit as white, granular texture within a water-soaked lesion. Typical signs and symptoms are shown in Figure 1. Each sporangium contains hundreds of smaller spores called zoospores. The sporangium will either directly germinate on new tissue to spread the infection, or in the presence of free water, will release zoospores, which have the ability to swim and actively seek out new hosts.

Figure 1. Signs and symptoms of Phytophthora capsici on A.) watermelon (photo: Jason Brock, University of Georgia,, B.) pickling cucumber (photo: A. Kness), C.) lima bean (photo: A. Kness), and D.) pepper with blighted plants in a field (photo: Howard F. Schwartz, Colorado State University,

Under favorable conditions and left unchecked, this disease can get out of hand quickly. If you’re planting susceptible crops, try to stay ahead of this disease, especially of you have a history of it in your field. A preventative, proactive management approach is best since rescue treatments are not effective. Below are some management options.

Crop rotation
If you have a history of this disease, rotate away from susceptible crops for at least 5 years; I realize that this many not always be practical. Agronomic crops, such as small grains, soybeans, and corn are good crops to work into a rotation.

Biological control
Some university research has indicated that biofumigant crops, such as mustards and rapeseed, especially those bred for high glucosinolate production, can reduce P. capsici inoculum in the soil and thus reduce incidence and severity of the disease. Achieving good biological control can be tricky since timing of flail chopping and incorporation of the crop is crucial. Mustards should be flail chopped two weeks prior to full bloom and incorporated into the soil via rototilling, heavy disking or plowing, within 20 minutes of chopping! Additionally, at least 0.5 inches of water should be applied after incorporation to seal the soil surface and provide sufficient moisture to completely synthesize the fumigation gases.

Water management
This disease requires a lot of water to infect a host, so water management is key. Do not over-irrigate and use drip irrigation when possible. Promote good soil drainage and install practices that eliminate pooling and facilitate water movement off-fields. Surface water can harbor P. capsici spores, so avoid irrigating from ponds or ditches.

Clean tools and equipment if you’ve been in an infested field. Harvest infested fields last to avoid spreading inoculum to new fields.

Chemical control
As mentioned earlier, oomycetes have cell walls made of cellulose, not chitin. Many fungicides penetrate and act on cell walls made of chitin, but not on cellulose. For this reason, many fungicides are ineffective at managing Phytophthora rot. Mefenoxam (Ridomil Gold) is an old chemical that still has good efficacy against P. capsici; however, resistant isolates are starting to appear. Additional fungicides that have good activity on P. capsici are cyazofamid (Ranman), oxithiopiprolin (Orondis products), and mandipropamid (Revus). It’s best to rotate chemicals and modes of action to avoid resistance. Fungicide applications should only be made if environmental conditions (outlined above) are favorable for disease development and you have a virulent pathogen in the field. Always read the label before applying any chemicals. For more information on chemical control, refer to the Mid-Atlantic Commercial Vegetable Production Recommendations.

1. Lamour, K. and Hausbeck, M. 2003. Effect of crop rotation on the survival of Phytophthora capsici in Michigan. Plant Dis. 87:841-845.
2. Lamour, K., Stam, R., Jupe, J., and Huitema, E. 2012. The oomycete broad-hostrange pathogen Phytophthora capsici. Molecular Plant Pathol. 13:329-337

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