Recommended fungicides include:
- Benomyl - various although most chocolate spot populations are resistant to benzimidazoles (i.e. benomyl)
- Chlorothalonil – ‘Bravo 500’, various
- Iprodione – ‘Rovral Flo’, 'Compass'
- Vinclozolin – ‘Ronilan FL’
- Cyproconazole – ‘Alto’,’Alto Elite’
- Tebuconazole – ‘Folicur’
- Fenpropimorph – ‘Corbel’
- Metalaxyl-M + chlorothalonil - 'Folio Gold'
- Azoxystrobin - 'Amistar'
- Metconazole - 'Caramba' (60g/l), 'Sunorg Pro' (90g/l)
- Pyraclastrobin + Boscalid - 'Signum'
Chlorothalonil is one of the cheapest fungicide options available and provides some of the best control of Botrytis when applied as a preventative fungicide. It will also control Ascochyta (although primary control of this disease should be through good seed screening and treatment) and suppress Downy mildew. Chlorothalonil should provide a base of preventative fungicide control with applications made as each layer of the canopy expands . This generally constitutes two or more usually three sprays throughout the season (Figure 3). Fungicide rates should be determined in relation to the persistence of control required. If an application is to precede another, rates can be reduced to 30 to 50 % of the full recommended rate. However, the final spray should be applied at or near full rates to ensure persistent control of disease throughout the seed filling period. Other fungicides are often added to chlorthalonil to improve preventative disease control. Metalaxyl-M ('Folio Gold') improves Downy mildew control (albeit at significant cost) whilst other fungicides are often added to improve the spectrum of activity or provide a degree of curative control. Some human consumption protocols place restrictions on the use of chlorothalonil, and depending upon target market, growers should confirm that protocols to which the crop is subject do not limit the products use.
Iprodione (‘Rovral’) and vinclozolin (‘Ronilan’) provide some curative control of chocolate spot. They are excellent fungicides for Botrytis providing preventative and some curative control. Both are persistent but iprodione responds to latex based adjuvants such as ‘Bond’ (Figure 1) whereas vinclozolin does not. Their weakness is that they are not truly systemic and thus are less suitable for the control of establishing disease. Both products are used for minority, high value crops and their cost has perhaps not fallen in line with more ‘commodity’ type products, making them less competitive in terms of cost effectiveness. Additionally, they have been superceeded by some of the newer fungicides.
Benomyl was also used to provide good control of chocolate spot but mbc resistance is now common and this fungicide offers little benefit. There are also significant health concerns over this product, and again protocols should be checked before use.
Adjuvants not only increase the level of control but also increase the persistence of control, which may be useful towards the end of the season during the long seed filling period.
These fungicides can be used in conjunction with chlorthalonil to provide season long control of chocolate spot and Ascochyta (Figure 2).
Tebuconazole will provide good control of chocolate spot in addition to rust control. Fully systemic, its ‘kick back’ can make it useful in dealing with diseases that are taking hold in a crop. It is often a useful addition to the final fungicide application when rust infection is likely. Cyproconazole is an excellent rust fungicide and can be applied in mixture with chlorothalonil as ‘Alto Elite’ (Figure 4). Metconazole ('Caramba', 'Sunorg Pro') has recently been recommended on beans for control of rust although no comparison has yet been seen between it and cyproconazole.
Azoxystrobin is a recent addition to the bean growers armoury of fungicides. A member of the strobulurin group it gives excellent results with:
- Chocolate spot
but moderate control against
- Downy Mildew
For general use an application of 'Amistar' 0.5-0.75 l/ha + 'Bravo' 1 l/ha is usually sufficient. The addition of chlorothalonil is usually cost effective in giving enhanced disease control (particularly chocolate spot) and is also good practice in terms of anti resistance strategy. Normal timing would be early flowering followed by mid-flowering 3-4 weeks later, although since both mix partners are anti sporulants best results will be obtained when applied at the earliest signs of visible disease. The mid flowering timing can be especially important for crops grown for certain markets if it coincides with 1st pod set, as this will may have quality implications.
A second strobilurin based fungicide has recently been recommended for use on beans. Pyraclastrobin is available in conjunction with Boscalid as ‘Signum’, for control of chocolate spot and rust. Boscalid has proven to be an excellent botrytis and ascochyta fungicide in other crops, whilst pyraclastrobin provides preventative chocolate spot and rust control. In trials ‘Signum’ has provided superior chocolate spot control over Ronilan + chlorothalonil, azoxystrobin + chlorothalonil and tebuconazole. (Figure 5 )
Additionally it possesses a shallow dose response curve and optimal control is achieved at 0.5l ha-1, equivalent to half rate. Application costs are thus equivalent to triazoles or azoxystrobin mixes but provide better disease control.
If rust or chocolate spot are already well established before spraying, tebuconazole is probably the best curative option.
However if downy mildew is the principal target (ie. becoming established in the crop or if the variety has a high susceptibility to the disease) it would be advisable to use 'Folio Gold' instead of 'Bravo 500'. A cheaper but preventative option to these metalaxyl products is sulphur, but this will necessitate frequent respraying (perhaps fortnightly) during the high risk period.
Although azoxystrobin is likely to justify itself in terms of disease control alone, it may give the grower the bonus of certain physiological effects that may result in prolonged green leaf retention (Figure 6).
Greening effect of Amistar
In summary, most likely fungicide choices will be between:
'Signum'} - standard broad spectrum preventative products
Tebuconazole - emergency curative treatment for Rust and Chocolate Spot
Cyproconazole - specific rust control
Metalaxyl - emergency curative treatment for Downy Mildew
Please note that the above is intended as a general guide and should not be taken as advice for specific situations. For this growers should contact their professional adviser, distributor or manufacturer. Always read the label before use.