- OOMYCETE DISEASES OF CUCURBITS 299
infection can result in sporangial production in 4 d at night tempera-
tures of 15◦C and day temperatures of 25◦C. Symptoms appear 3–12 d
after infection, depending on temperature, presence of free moisture,
and inoculum density. High temperatures (> 35 ◦C) are not favorable for
downy mildew development. However, if cooler nighttime tempera-
tures occur, disease development may progress.
- Disease Management.Effective management of downy mildew of
cucurbits requires integrated approaches of cultural practices, use of
resistant cultivars, and fungicide applications (Palti and Cohen 1980;
Sherf and MacNab 1986; Thomas 1996; Colucci and Holmes 2010). A
forecasting system has been developed at North Carolina State Univer-
sity to help monitor downy mildew outbreaks in cucurbit-producing
areas and track the movement of sporangia (www.ces.ncsu.edu/depts/
pp/cucurbit).
Cultural Practices.Cultural practices to minimize the incidence and
severity of downy mildew of cucurbits include maximizing the dis-
tance from potential sources of inoculum and reducing humidity in the
canopy (Palti and Rotem 1981; Thomas 1996). Increasing plant spac-
ing, which reduce canopy density and humidity, and avoiding overhead
irrigation are recommended. In greenhouse or other indoor production
situation, practices should be used to reduce the relative humidity as
well as the presence of free moisture on the leaves.
Host Resistance.Most commercial cultivars of cucumber and can-
taloupe, and some cultivars of squash and pumpkin, have some lev-
els of resistance to downy mildew (Thomas 1996), although they are
typically not high enough to completely control the disease. Resistance
may result in less severe disease, less sporulation of the pathogen on
infected leaves, and/or delay of disease onset. However, new strain of
P. cubensismay develop and overcome the existing resistance, as the
ineffectiveness of the existing resistance in cucurbits has already been
suspected and is under investigation (Holmes et al. 2015).
Chemical Control.Downy mildew of cucurbits can be effectively
controlled by using fungicides (Thomas 1996; Gisi 2002; Colucci
and Holmes 2010). Effective fungicides include ametoctradin +
dimethomorph (Zampro 525F), cyazofamid (Ranman 400SC), famox-
adone + cymoxanil (Tanos 50WDG), fluopicolide (Presidio 4SC),
mandipropamid (Revus 2.09SC), and zoxamide+mancozeb (Gavel
75DF). SinceP. cubensisis known to develop resistance to fungicides
very rapidly, mixing these fungicides with a protectant fungicide, such
as chlorothalonil or mancozeb, could increase efficacy of the fungi-
cide and delay resistance development in the pathogen (Colucci 2008).