Agriculture Reference
In-Depth Information
commercialized, but currently, we are unaware of any fungicide that could
effectively halt epidemics caused by metalaxyl-resistant strains under conditions
favourable to
P. infestans
growth and development. Thus it is important to have
protectant fungicide on plants prior to establishment of disease. Good coverage is
required for adequate suppression, and good coverage is most readily achieved with
relatively frequent application, particularly when plants are growing rapidly. It was
learned that, on average, not all sprays had an equal contribution to the suppression
of foliar disease (Shtienberg, 1989, 1994). Sprays applied early or mid-season had
larger effects on foliar disease than sprays applied late season. Management
programmes that include host resistance as a variable in addition to weather (Fry,
1977; Fry
et al
., 1983) can achieve considerable efficiencies in fungicide utilization.
Plants with field resistance slow pathogen growth rates. Unfortunately, most
cultivars that have market acceptance are not highly resistant to late blight.
Nonetheless there is diversity among commercial potato cultivars in terms of
resistance to late blight and these levels can be incorporated into an overall
management strategy. Cultivars with higher levels of resistance require less
fungicide than cultivars with lower levels of resistance (Fry, 1978).
Durable or polygenic resistance is sometimes interpreted to be synonymous with
intermediate resistance levels but cultivars ranging from complete susceptibility to
very highly resistant are possible (unpublished results). The problem has been that in
most locations these breeding lines have not yet become popular (an exception
appears to be Mexico, where more resistant cultivars have some market share)
(Grünwald
et al
., 2002b). Until these more resistant cultivars and breeding lines
become accepted, or until breeders create resistant potatoes that have more of the
qualities desired by the market place, integrated strategies will need to be used. One
possible strategy is to adjust the amount of fungicide to complement the level of
resistance in a cultivar (Fry, 1978). Another possible strategy that could be adopted
in certain locations is to combine field resistance with other cultivars in cultivar
mixtures. Cultivar mixtures have been evaluated for late blight management, but the
effects were small and not detectable in all locations (Garrett and Mundt, 2000;
Garrett
et al
., 2001; Andrivon
et al
., 2003).
The increased importance of late blight during the past few years has stimulated
plant molecular biologists to become interested in resistance to late blight. There has
been intense effort directed at cloning R-genes and in defining the signalling
pathways that lead to resistance. One potentially very important discovery has been
an R-gene from
S. bulbocastanum.
This gene has now been cloned, and it appears to
be similar in structure to other R-genes. The very significant difference is that this
gene seems to provide durable resistance to late blight, even when exposed for seven
years to highly variable populations of
P
.
infestans
(Song
et al
., 2003). Other efforts
have investigated the roles of genes coding for antimicrobial proteins and PR
(pathogenesis-related) proteins. When transformed into recipient plants, some of
these genes have conferred partial resistance in greenhouse experiments (Liu
et al
.,
1994; Wu
et al
., 1995).
The role of microbes in the population dynamics and survival of
P. infestans
is
largely unknown. Several reports of testing different organisms attempting to
biocontrol late blight have been published during the past decade. Biological control
