300 M. BABADOOST
When chemical control is used in combination with cultural practices,
host resistance, and disease forecasting, growers can reduce the pesti-
cide use and save money, and manage the disease with high efficiency.
Disease Forecasting.Early detection is the key to effectively managing
cucurbit downy mildew (Holmes et al. 2004; Colucci and Holmes 2010).
If fungicide sprays are not initiated in a timely manner, downy mildew
is difficult to manage. In 1998, Holmes and Main of North Carolina
State University developed a forecasting system to track the outbreaks
of cucurbit downy mildew, which is available online at http://www.ces.ncsu.
edu/depts/pp/cucurbit (Holmes et al. 2004).
C. Pythium Diseases
Pythium diseases include damping-off, root rot, and cottony leak (Got-
tlieb and Butler 1939; Sitterly and Keinath 1996). Pythium damping-off
and root rot are caused byP. ultimum,P. aphanidermatum,P. irregu-
lar,andP. myriotylum.Pythium cottony leak is caused byP. anandrum,
P. aphanidermatum,P. myriotylum,P. oligandrum,P. periplocum,and
P. ultimum.
- Symptoms. Damping-off and Root Rot.Damping-off and root rot of
cucurbits, caused byPythiumspecies, have been reported from vari-
ous locations in the United States, Canada, and several other countries
(McKeen and Thorpe 1968; Gubler and Davis 1996b). These pathogens
may cause pre- or post-emergence damping-off. In seedlings, a watery
rot develops in the taproot and hypocotyl at or near the soil line, which
results in seedling death (damping-off). Chlorosis of cotyledons and true
leaves may occur prior to seedling death. The root rots caused by dif-
ferentPythiumspecies are typically similar.
The numerous root rots caused by differentPythiumspecies are simi-
lar. Initially, feeder roots are infected and killed, and soon after, necrotic
lesions, 0.3–2.0 cm long, develop on lateral roots. Roots may have one
to several lesions. As the disease progresses and size of the lesions
increase, the plant may begin to wilt. Mature plants show symptoms
of root and crown rot. The crown leaves often become chlorotic. Necro-
sis soon follows, as the symptoms gradually move outward, toward the
runner tips, and the plant dies (Gubler and Davis 1996b).
Cottony Leak.Cottony leak, or Pythium fruit rot, is a common dis-
ease in most of cucurbit-growing areas. Water-soaked brownish lesions
develop on fruit, which results in a rapid soft rot of fruit (Sitterly and
Keinath 1996). The rotted areas quickly become covered with cottony
mycelium, particularly under humid conditions. In most cucurbits, the