DISEASE CONTROL

Rust on Dry Beans

Rust control in 1995 was generally better than in 1994, with
fewer losses in most areas. Some of this may have been due to
growers tank mixing a rust fungicide with a white mold fungicide
in anticipation of a rust outbreak, rather than waiting for rust
detection.  

Data from our grower survey indicates that some growers may not
appreciate the significance of rust, however. Rust can cause
complete crop failure, and in fact did so in some fields in 1981.
In our 1995 survey growers indicated that white mold was the
worst disease on about 70% of their acres. Rust was not ranked
that often as even one of the three worst diseases.  

Several sources of information indicate that although rust
control was generally good, some growers did not recognize the
extent of rust in their fields and they were not detecting rust
soon enough. In most cases, poor rust control resulted from late
application. Both chlorothalonil (Bravo, etc.) and maneb provide
good rust protection, but are protectants; they protect against
infection but do not stop established infections. Tilt, which was
available last year under a section 18, has up to four days of
post infection ("kick-back") activity; however, if it is applied
more than four days after infection, it will not stop infections.
Tilt is most effective if used as a protectant. There is also
less chance of fungicide resistance developing if Tilt is used as
a protectant. The status of Tilt for the 1996 growing season is
still unclear: it still is not registered. A section 18 has been
requested, but not granted as yet.

Rust developed within a few days of application in some fields.
Rust takes from seven days at warm temperatures to 10 days at
cooler temperatures to develop from initial infections to
spore-producing pustules. No product will stop rust when the
infections are within a few days of breaking through the
epidermis. Once rust is established in other fields in an area,
it is time to quit scouting and start spraying. In past years
growers who waited too long had severe rust develop a few days
after they sprayed. I suspect that large numbers of spores blew
in from nearby fields, the pustules seen in a field were but the
first visible infections, and that many others were only a few
days from erupting through the surface of the leaf. 

Growers should scout carefully early in the season, taking
special care to check near last year's fields and along shelter
belts. Once rust is established in an area, it is time to spray.
Most pinto varieties are susceptible to rust. Chase, however, is
resistant, and Hatton is moderately resistant. Most navy
varieties are moderately resistant, and most kidney varieties are
highly resistant to the current races of rust. All beans should
be scouted, however, because we never know when a new race will
arise that can attack a previously resistant variety. If you see
higher levels of rust than you would expect on a moderately
resistant or resistant variety, please contact Art Lamey (tel.
701-231-7056; FAX 701-231-7851; e-mail alamey@ndsuext.nodak.edu)
and also collect samples so that we can make a race
determination. Keeping track of new races is extremely important
to rust management in the region.

(Submitted by: Art Lamey, NDSU Extension Plant Pathologist)