Induced resistance for plant disease control - Disease Control in Crops - page 71

lesion diameter was reduced by ca 1828% on fruit that were dipped for 10 minutes,
2 days before inoculation.
Foliar applications of harpin, in combination with chlorothalonil and azoxystrobin,
were evaluated against downy mildew ( Pseudoperonospora cubensis ) and gummy stem
blight ( Didymella bryoniae ) on 'Athena' melon (Keinath et al ., 2007a). Harpin did not
control either disease but was associated with a 1012% increase in fruit yield in one year
out of two. The same authors conducted 12 fi eld trials between 2002 and 2004 in which
they evaluated harpin, ASM and Reynoutria extract (Milsana ® ), in combination with
chlorothalonil, for their ability to control gummy stem blight and Alternaria leaf blight
( A. cucumerina ) in 'Athena' melon (Keinath et al. , 2007b). Results indicated that harpin
and Reynoutria extract could be alternated with weekly chlorothalonil applications with-
out loss of disease control. ASM was, however, less effective and had a negative effect
on yield. Scheduling treatments according to the weather-based application programme
(Melcast) generally reduced the number of spray applications, but did not always provide
as effective disease control as the standard schedule, except when disease severity was
low. It was concluded that, to reduce the amount of fungicide applied, growers could use
reduced-risk fungicides (such as activators) in rotation with a reduced rate of fungicide on
a weekly basis. This offers the potential to use compounds with shorter re-entry intervals
compared with chlorothalonil (48 hours) and mancozeb (5 days) during the vital harvest
period. This is particularly important for a crop such as melon, since harvest can be pro-
tracted and may last between two to four weeks.

4.2.1.8 Grape

Grey mould ( Botrytis cinerea ), powdery mildew ( Uncinula necator ), and downy mildew
( Plasmopara viticola ) can cause signifi cant economic damage in vineyards worldwide.
Increasingly stringent regulations governing chemical residues on grapes and in wines are
restricting fungicide control options in conventionally managed vineyards. For example,
in New Zealand and Australia, preharvest intervals have been extended for several highly
effective fungicides. Similarly, changes are taking place in organic production in Europe
where initiatives are being funded to fi nd replacements for copper fungicides that are tra-
ditionally used to control downy mildew in grapevine. Resistance activators are amongst
the options that are being considered to supplement or replace fungicides in both conven-
tional and organic viticulture.
Ten fi eld applications of 0.2% SA reduced downy mildew severity on leaves and on
berries by ca 30% and ca 40%, respectively (Kast, 2000). However, SA can be phyto-
toxic and was reported to retard berry ripening in Shiraz grapes (Kraeva et al. , 1998).
Regular applications of 5-chlorosalicylic acid (5CSA) provided control of botrytis bunch
rot on Chardonnay grapes that was equivalent to that achieved by fungicide application
(Reglinski et al., 2005). However, as with SA above, 5CSA was phytotoxic and caused
leaf chlorosis and a reduction in berry weight. Furthermore, three applications of 5CSA
on 'Cabernet Sauvignon', between fruit-set and harvest, caused a decrease in wine quality
(Duxbury et al ., 2004). Three applications of ASM after veraison induced resistance to
B. cinerea in Merlot bunches (Iriti et al., 2004). None of the bunches from ASM-treated
vines exceeded 25% disease severity whereas over 90% of untreated bunches showed a
disease severity of >50%. ASM has also demonstrated effi cacy against downy mildew