New Developments in Managing Phytophthora Blight in Cucurbit Crops
Margaret Tuttle McGrath
Phytophthora blight (aka Phytophthora fruit and stem or crown rot) continues to be a challenge to manage, especially in a wet year. Fortunately there are new management tools. Ranman is a new addition to the fungicide arsenal. Plus there are new pumpkin varieties with hard rinds. While there have been no reports of Phytophthora blight affecting beans in NY, growers need to be aware that recent research in MI, where beans have been plagued since 2003, demonstrated that all 12 types of beans tested were susceptible to strains of the pathogen that were also able to infect cucumber.
Symptoms of Phytophthora blight on cucurbits include crown rot, tip blight, leaf spots, and fruit rot. Management focuses on preventing the pathogen from being moved into a new field and managing soil moisture to avoid saturated conditions that favor disease onset. Prevention is very important because Phytophthora blight is difficult to control once it starts, and after it has occurred on a farm it is challenging to continue growing susceptible crops without Phytophthora blight occurring, even in fields with no previous history. Additional information on symptoms and management can be found at:
Pumpkins with hard, gourd-like rinds or shells were shown to be less susceptible to Phytophthora fruit rot when mature than pumpkins with conventional, softer rinds through research conducted with Lil’Ironsides and Apprentice (HMX 5682) at LIHREC. Iron Man (HMX 2690) is another new variety with the gene for hard rind from Harris Moran. Degree of control obtainable with hard-rinded pumpkins is high: proportion of fruit developing Phytophthora fruit rot when evaluated in 1997 and 1998 was 2 and 29% for Lil’Ironsides, respectively, 2 and 12% for Apprentice compared to 25-42% and 56-71% for horticulturally similar pumpkins with conventional rind. Rockafellow is also described as having a hard rind. Cannon ball has a tougher skin than conventional pumpkins and thus may also be less susceptible to Phytophthora fruit rot. Cannon ball, Iron Man, and Rockafellow also have resistance to powdery mildew.
Descriptions of pumpkin varieties with hard shell or tough skin:
Apprentice 1 lb fruit, 4-5 in diameter, round, dark orange, bush type
Lil' Ironsides 2 lb fruit, 6 in diameter, round flat, dark orange, vine type
Iron Man 3-4 lb fruit, 6-7 in diameter, round, dark orange, vine type, PMR
Rockafellow 2.5-3 lb fruit, elongated oval shape, very smooth, semi-bush type, PMR
Cannon Ball 5 lb fruit, 7-8 in diameter, round, dark orange, semi-bush type, PMR
These varieties will be evaluated on Long Island in 2006 along with several experimental lines producing small, medium or large fruit with a hard shell.
Fungicides can be a valuable component of a Phytophthora blight management program and fortunately several products have been registered recently. But it is important to realize that none of the fungicides and experimental materials tested to date in university efficacy studies have worked sufficiently well that they could be relied on as the sole management practice for this disease. Start fungicide applications for Phytophthora blight before symptoms have been observed and when plants are young (4-6 inches high or two-leaf stage suggested), repeat at 7- to 10-day intervals or when conditions are favorable for disease. These fungicides are also labeled for downy mildew. Unfortunately they are not effective for powdery mildew.
Ranman 400SC (EPA Reg. No. 71512-3; FRAC Group 21 fungicide; ISK Biosciences Corp) contains cyazofamid, an active ingredient effective for oomycetes, the group of pathogens responsible for late blight and downy mildew as well as Phytophthora blight. It was registered in NY on 3 March 2006 for use on tomatoes and potatoes in addition to cucurbit crops. It has limited systemic activity. Due to potential for resistance to develop, it is critical to implement resistance management strategies: apply Ranman both tank-mixed with a protectant fungicide and in alternation with other labeled fungicides that have a different mode of action (different FRAC Group number). Follow these strategies beginning the first year of use because the main goal of resistance management is to delay its development, rather than manage resistant strains, and furthermore these strategies are specified on the label which is a legal document. Use an organosilicone surfactant such as Silwet L-77 with Ranman. For Phytophthora blight, apply Ranman at 2.75 fl oz/A beginning before symptoms for a maximum of 6 applications. Cost is about $16.75/A/application. It is labeled at a lower rate for downy mildew (2.1–2.75 fl oz/A). The minimum interval is 7 days, REI is 12 hr, and the PHI is 0 days.
Other fungicides labeled for Phytophthora blight to use in alternation with Ranman are Forum, Gavel, Tanos and phosphorus acid fungicides (ProPhyt, Phostrol and Fosphite). Gavel and Tanos are available for use in NY for Phytophthora blight under FIFRA 2(ee) Recommendation, a copy of which must be in the applicators’ possession when either of these are used in NY. A copy (tif file) can be obtained at
There are no protectant fungicides specifically labeled for Phytophthora blight to use in tank mixes. A 2(ee) has been approved for several copper hydroxide fungicides (Kocide, Champ, and Champion) and the copper sulfate fungicide Cuprofix Disperss. This request to add use for Phytophthora was made because in NY the target pest must be specified on the label, new fungicides being labeled for Phytophthora blight are recommended used in a tank-mix with other labeled fungicides (while not specified on most labels, the intent is that this be a protectant fungicide), and none of the protectant fungicides labeled for use on cucurbit crops include Phytophthora blight on the label since they do not provide sufficient control of this disease when used alone. Most evaluation studies conducted at University facilities of these new fungicides have tested them as tank mixes with copper fungicides. This 2(ee) recommendation will be posted soon at the above web site. A 2(ee) is pending for maneb and mancozeb containing products, which are tank-mix partners specified on the Tanos label. If NYS DEC approves this 2(ee), it will be posted at the above web site.
Forum 4.18 SC (FRAC Group 15) is a new formulation of dimethomorph replacing Acrobat 50WP, which was registered in 2005 for use in NY. It is labeled for use on all cucurbit crops at 6 oz/A every 5 to 10 days, depending on disease pressure, beginning when plants are 4-6 inches high for a maximum of 30 oz or 5 applications. Cost is about $8.90/A/application. Label specifies that it must be tank-mixed with another fungicide and applied no more than twice before alternating with another fungicide. Forum is also registered for use on lettuce, bulb vegetables, potato, eggplant, pepper and tomato. REI is 12 hr and the PHI is 0 days.
Tanos (EPA Reg. No. 352-604), containing famoxadone (Group 11) and cymoxanil (Group 27), is labeled for use at 8 oz/A for a maximum of 4 applications. Cost is about $10.30/A/application. It must be tank-mixed with a copper fungicide and a fungicide containing maneb or mancozeb. Follow a strict alternation with no consecutive applications of Tanos. REI is 12 hours and PHI is 3 days. Other labeled crops include head lettuce, pepper, potato, and tomato.
Gavel 75DF (EPA Reg. No. 62719-441) contains mancozeb and zoxamide, a new active ingredient that specifically targets Phytophthora and related fungi. Gavel can be used on cucumber, melon, summer squash, and watermelon but not on pumpkin because it contains mancozeb. It is labeled on cucurbits for downy mildew, Alternaria leaf spot, and fruit and stem rot. The FIFRA 2(ee) approved in 2004 was needed because the label has just fruit and stem rot without specifying the pathogen Phytophthora. Gavel is also registered for use on tomato and potato. It is labeled for use at 1.5–2.0 lb/A every 7 to 10 days or when conditions are favorable for disease for a maximum of 8 applications. REI is 48 hr and the PHI is 5 days. Cost is about $7.75-10.30/A/application.
New phosphorous acid fungicides (ProPhyt, Phostrol and Fosphite)(Group 33) are more effective than Aliette. They have a 4 hr REI and can be applied to all cucurbits at 2-6 pt/A on a 7-14 day interval up to 6-7 times/crop. PHI is 0 days. Cost is about $4.65-14.00/A/application for Fosphite. Phosphite ion, the active ingredient for these fungicides, effects fungal pathogens directly and promotes the plant’s defense system. In addition to foliar applications, ProPhyt is now labeled for use as a drench treatment to transplants before transplanting or as an in-furrow drench at planting. Research is being conducted on applying ProPhyt through drip irrigation during the season. These fungicides are also labeled for use on asparagus, crucifers, leafy vegetables, onion, potato, and tomato.
These new fungicides have performed well in recent fungicide efficacy experiments conducted by University scientists. For example, in an experiment conducted in 2005 in IL with processing pumpkin, the most effective fungicide program was Tanos + Manex + Kocide (copper) alternated with Acrobat + Kocide, providing 82% control of Phytophthora fruit rot. Phytophthora fruit rot was reduced 75%, which was not significantly different than the best fungicide program, with Phostrol + Cuprofix alternated with Acrobat + Cuprofix. Control was also 75% with a 3-way alternation of Tanos + Manex + Kocide alternated with Gavel + Kocide and with Acrobat + Kocide. Gavel + Cuprofix alternated with Cuprofix provided 66% control. Remember that Gavel is not yet registered for use on pumpkins. Ranman + Cuprofix (copper) alternated with Topsin M (thiophanate methyl) + Cuprofix provided 62% control.
For an intensive fungicide program for Phytophthora blight in cucurbits, begin with a drench treatment of ProPhyt to transplants, then alternate among the following applied to foliage (5 to 10 day spray interval; can be extended under dry conditions):
1. Forum + copper fungicide when plants are 4-6 inches high or at 2-leaf stage.
2. Ranman + copper fungicide.
3. ProPhyt, Phostrol or Fosphite
4. Tanos + copper fungicide + maneb or mancozeb
5. Gavel for cucurbit crops other than pumpkin.
Please Note: The specific directions on fungicide labels must be adhered to -- they supersede these recommendations if there is a conflict. Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.