PCC March 2019

Kasugamycin is applied at 64 fl oz per
100 gal or 100 ppm. Thus, the labeled
rate is kasugamycin-mancozeb mixtures
are approximately 20X of the MIC value
for Xaj. Because of the short residual
activity and a moderate buffering residue
(20X), the rotation needs of bactericide
mixtures containing kasugamycin
described above need to be applied in 7-
to 10-day intervals (Figure 3).
Kasugamycin and Resistance
Resistance is a relative term indicating
a change in sensitivity to an inhibitory
compound. A moderately high MIC for
a bactericide does not mean that the
pathogen is resistant. We have conducted
baseline studies with kasugamycin,
kasugamycin-copper, and kasugamycinmancozeb
for Xaj with MIC values of
20, 8.3, 5.3 ppm, respectively (Figure 2,
see page 14). This was done before the
bactericide was registered in California
to determine any change in sensitivity
after registration and commercial usage.
To date, resistance has not been found
and isolates evaluated are all within
the baseline distributions. Still, with a
single site mode of action compound
such as kasugamycin, there is a risk for
selecting resistant sub-populations of
the pathogen especially when resistance
management strategies are not employed.
This is the reason why we developed the
mixture-rotation programs suggested
above.
Conclusions
The integration of bactericides with
different modes of action and application
strategies of rotations of mixtures of
bactericides with different modes of
action with forecasting tools such as
XanthoCast (http://www.agtelemetry.
com/) should provide the stewardship
necessary for having the tools available
for managing walnut blight for years
to come. The hope with the Kasumin
registration is to provide resistance
management and prevent or reduce the
risk of resistance to copper-mancozeb
while new approaches can be developed
and integrated to protect all of these
compounds. Walnut blight is the most
serious disease impacting growers
in California and multiple tools like
kasugamycin, copper, and mancozeb
need to be available to maintain a
successful industry.
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Treatment (Rate of ai oz/A)
Control
ATD - 12.9
Kasugamycin - 1.3
Kasugamycin + ATD - 1.3 + 12.9
Copper + Mancozeb - 24 + 28.4
Kasugamycin +Copper - 1.3 + 24
Kasugamycin + Mancozeb - 1.3 + 28.4
Copper - 24
0
d
d
cd
bc
bc
b
bc
2 4 6 8 10
Disease incidence (%)
a
Figure 3. Efficacy of
treatments for managing
walnut blight.
Treatments applied
using an air-blast
sprayer (100 gal/A). The
walnut blight pathogen
was sensitive to copper.
Disease incidence is
the number of diseased
nuts per 100 nuts
evaluated. ATD = amino
thiadiazole, an experimental
bactericide. Bars
followed by the same
letter are not significantly
different.
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