298 M. BABADOOST
Table 6.1. Interactions of cucurbit hosts with pathotypes ofPseudoperonospora
cubensisadapted from Thomas et al. (1992).
PathotypezHost 12345
Cucumis sativus +++++
C. melovar.reticulatus +++++
C. melovar.conomon −++++
C. melovar.acidulous −−+++
Citrullus lanatus −−−++
Cucurbitaspp. −−−−+
z+, highly compatible host–pathogen interaction;−, incompatible or very slightly compatible host–
pathogen interaction.
family has been reported (Thomas and Jourdain 1992; Lebeda and
Widrlechner 2003). Thomas et al. conducted a study in 1987 that sum-
marized five distinct pathotypes (Table 6.1) (Thomas 1996). Recent
studies suggest that more pathotypes ofP. cubensismay exist (Colucci
2008).
- Disease Cycle and Epidemiology. Pseudoperonospora cubensissur-
vives on cucurbit hosts either in areas with climates that permit their
growth throughout the year or on cucurbit crops produced in green-
houses or other protected cultures (Thomas 1996). The source of pri-
mary inoculum is windblown sporangia.
Downy mildew of cucurbits is a polycyclic disease (Colucci and
Holmes 2010). Once sporangia land on a susceptible host, free moisture
is required for each sporangium to release zoospores. Free moisture is
also important for zoospore movement, germ tube development, and
penetration of host tissue by the germ tube. Optimum temperature for
zoospore release depends on the duration of the leaf wetness period. If
the host leaf remains wet for only 1 hr, the minimum temperature for
zoospore release is 10◦C, and the greatest amount of spore release occurs
at 20◦C. If the host leaf remains wet for 2 hr, the minimum tempera-
ture is 5◦C, and the greatest amount of zoospore release occurs at 15◦C.
The optimum temperature for zoospores to form cysts is 25◦C (Thomas
1996).
Optimum temperature for germination of the cysts is 25◦C (Thomas
1996). With very high inoculum levels, some successful penetrations of
germ tubes occur when free moisture is present for as little as 2 hr. Once
the germ tube has penetrated the substomatal chamber, the presence of
free moisture is no longer required for the infection process to proceed.
Haustoria form in about 4 hr. With an 18-hr photoperiod, successful