How to control Phytophthora

Phytophthora root rot is more difficult to control than Pythium and Rhizoctonia. While Phytophthora can contaminate greenhouse floors and used flats, it can also come in on rooted cuttings or prefinished plants. Early on, infected plants may not show symptoms until the infection is established or the plant becomes stressed, such as from over- or under-watering.

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What to look for

Inspect the roots of all plants brought in for finishing. Roots should be white. If they aren’t, send samples to a diagnostic lab to determine if a pathogen is present.

The sooner that Phytophthora is found, the better the chances of treating it before an outbreak occurs. Once this pathogen has been introduced into the greenhouse, it moves through water splashing or by recirculating irrigation water.

Water is Phytophthora’s driving force. Under warm, wet conditions many spores develop on infected plants and lead to a rapid buildup of the disease.

While the best way to control Phytophthora is to keep the pathogen off the production site, what can you do once you have it? Start by removing visibly diseased plants. Sanitation, such as removing plant debris and disinfesting pots and production surfaces, is also important.

Fungicide treatment is generally required to ensure that the Phytophthora has been brought under control and won’t cause further plant harm. Drenching plants with mefenoxam (Subdue MAXX) has long been an industry standard for this disease and continues to offer excellent control. However, no fungicide should be relied on exclusively for disease management and the industry needs additional tools that can be used in a program with Subdue MAXX.

Testing current, new products

Studies conducted at Michigan State University in partnership with the USDA IR-4 Project and fungicide manufacturers included registered and unregistered fungicides for their ability to control Phytophthora on snapdragon. Controlling Phytophthora on herbaceous crops that are tender plants is especially challenging.

In our trials, all plants infected with Phytophthora that did not receive a fungicide treatment died. Subdue MAXX performed well and kept all plants healthy. Other registered products, including Stature DM, Terrazole 35WP and Insignia 20WG, looked promising and treated plants were significantly healthier than those that were not treated. Truban 30WP did not perform especially well in these trials with some treated plants dying and others showing wilt symptoms.

A new fungicide called FenStop (available soon) performed very well. Other products such as NOA 446510 (Syngenta), V-10161 (Valent), and V-10162 (Valent) that are not labeled yet also looked very good when tested.

It is exciting for the industry to have so many promising products on the horizon to assist growers in managing Phytophthora.

This research was funded by the USDA-ARS as part of the Floriculture and Nursery Research Initiative (Agreement 59-1907-5-553) and the IR-4 Project.

^*Column means with a letter in common are not significantly different (Student-Newman-Keuls; P=0.05).

^**These products are not registered.

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Check label for application intervals. Application intervals can be shortened when different fungicides are used in alternation as part of an overall treatment program.

- Mary Hausbeck