Disease Management

Cucumber Powdery Mildew

Powdery Mildew of Cucurbits

Powdery mildew is a major production problem in cucurbit crops in all parts of the world. All cucurbits are susceptible, but the disease is less common on cucumber and melon due to the prevalence of resistant cultivars. Yields are reduced by a reduction in the number and/or size of fruit. Fruit quality can also be adversely affected by sunscald (due to defoliation), incomplete ripening, reduced storability (winter squash), and poor rind quality or discolored handles (pumpkins). In addition, infection by Powdery mildew predisposes plants to other diseases (Gummy stem blight). Spaerotheca fuliginea and Erisyphe cichoracearum are the two most common fungi causing Powdery mildew on cucurbits. S. fuliginea is more prevalent and more aggressive; E. cichoracearum is found mainly in the cooler spring and early summer months. Pathogenically specialized races of S. fuliginea have been reported.

Symptoms occur on both leaf surfaces, stems, and petioles as white, powdery fungal growth. Symptoms develop first on older leaves, shaded lower leaves, lower leaf surfaces, and on older, fruit bearing plants. Infected leaves shrivel and die; plants may senesce prematurely. The pathogens are obligate parasites and cannot survive in the absence of living hosts; initial inoculum for the Northeast is most likely airborne conidia originating in southern states. Other possible sources include greenhouse grown cucumbers and alternate hosts. Under favorable conditions, Powdery mildew develops rapidly; the time between infection and symptom expression can be as short as 3 days and many spores are produced. Conditions favoring infection include a dense plant canopy, low intensity light, high nitrogen fertilization, and high relative humidity (although infection can occur at relative humidity of less than 50%). Optimum temperatures for disease development are 68-80° F; infection can occur between 50-90° F. Temperatures of 100° F or above stop Powdery mildew development.

Management

• Plant resistant varieties where available. Resistant varieties of squash and pumpkin are under development.
• Separate successive cucurbit plantings physically to prevent older plants from serving as an inoculum source for main crop.
• Scout fields regularly (particularly lower leaf surfaces) and apply fungicides early in disease development. Powdery mildew cannot be effectively controlled by fungicide applications after the disease is established.
• Powdery mildew develops best on the lower leaf surfaces; thus a successful fungicide program requires controlling the pathogen on both leaf surfaces. An important component of fungicide programs are materials which can move to the lower surface (systemic or translaminar). Systemic fungicides, due to their single site mode of action, are prone to resistance development and the powdery mildew fungi have demonstrated the ability to develop resistance to these fungicides: (benzimidazoles (Topsin M) and strobilurins (Flint, Cabrio, Amistar). Managing resistance is an important consideration in selecting a fungicide program. Current recommendations for managing resistance consist of an alteration of effective high-risk materials of two or more chemical classes at 7 day intervals, with a protectant fungicide included in every application. A protectant fungicide has multi-site activity, low resistance risk, and will control strains resistant to the systemic chemical.

Chemical recommendations:

Systemics:
myclobutanil (Nova): 5 oz/A (0 dh, REI 24 h, Group 3). Observe a 30 day plant-back interval.

pyraclostrobin plus boscalid (Pristine): 12.5 to 18.5 0z/A (0 dh, REI 12 h, Groups 11 & 7). Do not alternate with other strobilurin fungicides (Group 11) like Quadris, Flint, or Cabrio.

Quinoxyfen (Quintec): 4 fl oz/A (3 dh, REI 12 h, Group 13). Section 18 New York State, not registered in Massachusetts, but will make good rotational partner for Pristine and Nova. Do not make consecutive applications of Quintec.

Protectants:
cupric hydroxide (Kocide 4.5 LF): 1.3 pt/A (0 dh, REI 24 h, Group M1).

sulfur (Microthiol Disperss): 5-10 lb/A (0 dh, REI 24 h, Group M1). Do not apply when temperatures exceed 90° F as plant injury may occur. Do not use on sulfur sensitive varieties.

References:
McGrath, M.T. 2005. Guidelines for Managing Cucurbit Powdery Mildew with Fungicides in 2005. http://www.vegetablemdonline.ppath.cornell.edu

McGrath, M.T. 1997. Powdery Mildew in Cucurbit Crops. http://www.vegetablemdonline.path.cornell.edu

Articles Available on Cucumber Powdery Mildew:

• Cucurbit Diseases and Fungicides: How to Choose a Spray Schedule