Fungicides
Inputs/Production
Soybean rust – managing the risk
of resistance to triazole fungicides
PROF MD LAING AND DR PM CALDWELL, DISCIPLINE OF PLANT PATHOLOGY,
SASA, UNIVERSITY OF KWAZULU-NATAL
In the absence of resistant cultivars, the South African soybean industry
is totally vulnerable to Asian soybean rust, caused by the fungus
Phakopsora pachyrhizi.
Asian soybean rust is very dependent on fungicides to protect the crop.
Initially, crop losses were managed in South Africa through emergency
fungicide registrations followed by an intensive research programme.
Over the past seven years, numerous systemic fungicides have been
registered for the control of Asian soybean rust in South Africa. At
present, Asian soybean rust can be controlled with one to two sprays
applied at flowering, using the registered fungicides on the market.
Because of their high level of efficacy, the triazoles have been used widely
to control Asian soybean rust. At present, no resistance by
P. pachyrhizi
to triazole fungicides has been detected in South Africa.
However, if producers continue to use triazoles without alternating with
other registered fungicides with different modes of action, then this
aggressive pathogen is very likely to develop resistance to triazoles. The
challenge therefore facing South African soybean producers is to use
these fungicides to best effect without running the risk of losing them
to resistance.
To slow down resistance development and to extend the life of a
fungicide, it is advisable to follow the recommendations of the Fungicide
Resistance Action Committee (FRAC), specifically:
Rotate fungicides with different modes of action, e.g. triazoles/
strobilurin/triazole or triazole/protectant/triazole.
Use a premix, e.g. triazole plus strobilurin or protectant.
Reduce exposure to two applications of each systemic fungicide
group per season.
Use a full dose, not a reduced dose of the fungicide.
Resistance to triazoles is based on the accumulation of several mutations.
This is described as “continuous selection” or “shifting”. Resistance
to triazoles is therefore stepwise, characterised by a gradual loss of
efficacy under field conditions. Total resistance to triazoles has rarely
been observed. This relatively slow development of resistance provides
opportunities for effective resistance management programmes.
The use of, for example, a strobilurin-triazole mixture, is the major
strategy, promoted by the agricultural industry, for reducing risk of
resistance towards both fungicide groups. These two active ingredients
are complimentary in their action because strobilurins inhibit fungal
respiration and consequently inhibit spore germination, whereas triazoles
inhibit germ tube elongation, fungal penetration and mycelial growth.
However, the choice of dual product fungicides with activity against rusts
is limited in South Africa. Unfortunately, carbendazim is not effective
against rust, so the mixing of triazoles with carbendazim does not protect
against resistance development of rusts, the way it does against grey leaf
spot of maize (
Cercospora zeae-maydis
).
Triazole fungicides remain vital to the economic production of soybeans
in South Africa, playing a vital role in controlling Asian soybean rust.
It is therefore crucial that all members of the soybean community
recognise the vulnerability of the family of fungicides and that soybean
producers commit to the FRAC recommendations to manage resistance
development, by mixing fungicide groups, alternating groups, only
spraying triazoles twice a season and using full recommended doses.
23
January 2014
Product name
Reg. No. L Act
No. 36 of 1947
Active ingredient
Concentrate
Hazard
classification
Company
ABACUS
8048
Pyraclostrobin/
epoxiconazole
125 g/l
Harmful
BASF
Bayfidan® 250DC
3179
Triadimenol
250 g/l
Harmful
Bayer
CAPITAN® 250EW
6062
Flusilazole
250 g/l
Harmful
DuPont
D-Zole 250 EC
7923
Difenoconazole
250 g/l
Harmful
Universal Crop Protection
Folicur® 250EW
3857
Tebuconazole
250 g/l
Caution
Bayer
IMPACT
4088
Flutriafol
125 g/l
Caution
Cheminova
Lyric® 250 EW
7885
Flusilazole
250 g/l
Harmful
DuPont
Lyric® C
7648
Carbendazim/
flusilazole
125/250 g/l
Harmful
DuPont
PRONTO 250 EC
7602
Difenoconazole
250 g/l
Caution
Meridian Agritech
PUNCH® C
3626
Carbendazim/
flusilazole
125/250 g/l
Harmful
DuPont
PUNCH®-XTRA
4243
Carbendazim/
flusilazole
250/125 g/l
Harmful
DuPont
Scope 250
7942
Difenoconazole
250 g/l
Harmful
Villa Crop Protection
SCORE® 250EC
5132
Difenoconazole
250 g/l
Caution
Syngenta
SHAVIT 25EC
5601
Triadimenol
250 g/l
Harmful
Makhteshim-Agan
Tebucure
7992
Tebuconazole
250 g/l
Caution
Universal Crop Protection
TRISTAR EC
6905
Triadimenol
250 g/l
Harmful
Meridian Agritech
Source: Registrar, Act No. 36 1947, personal communication November 2006
PUNCH®, CAPITAN® and LYRIC® are registered trademarks of DuPont or its affiliates
Bayfidan® and Folicur® are registered trademarks of Bayer CropSCience Ag, Germany
SCORE® is a registered trademark of the Syngenta Group Company
TABLE 1: CHEMICALS REGISTERED FOR SOYBEAN RUST CONTROL.
