November 2006. Samoa asked for information on the Strobilurin group of fungicides in regard to the management of banana black leaf streak (BLS). Punch and Tilt are no longer effective (but see below), although they were 20 years’ ago. So the question is, how to prevent the build up of resistance to maintain the usefulness of the product.
The Strobilurin group of fungicides is highly effective against certain types of fungi. They are also good in that they are considered a “reduced risk” fungicide – they are meant to be less harmful to humans and to the environment than alternative fungicides (although some have been found to be toxic to highly toxic in aquatic ecosystems).
The following questions should be asked when considering which fungicide to use:
1. What is the target fungus? It is important to determine if the fungus you are wanting to manage is a member of a group that can be controlled by the fungicide you are considering. In the case of strobilurins, they have an extremely broad spectrum; however, there are exceptions so check carefully. Strobilurins are better protective fungicides than curative; this means they can prevent infections, but once the infection has occurred, they aren’t so useful. This is due to the way they bind with the waxy cuticle of leaves.
2. Who are the target users? Strobilurin fungicides are not cheap, so are they a viable option for Pacific Island farmers?
3. Can the fungicide be readily sourced? If supplies of the fungicides are irregular, then that can be a problem for farmers. It is good to work with the local pesticide dealers, if these (strobilurin) fungicides are to be promoted. There is also the question of their registration.
4. Do we really need to use strobilurins when other fungicides will do?
5. Should the fungicide be used in rotation? This brings up a very important issue to do with the strobilurins. There is a high risk of development of resistant populations with repeated use of these fungicides. A resistance-prevention strategy needs to be considered if there is going to be widespread use of strobilurins. Strobilurin fungicides display qualitative resistance, meaning that the pathogen is either highly sensitive or highly resistance. With qualitative resistance, control cannot be regained by applying the fungicide more frequently and/or at a higher rate or by switching to a more active fungicide in the same chemical class, in contrast with quantitative resistance. All strobilurins will be affected by the resistance.
There are some excellent information resources on BLS control on the INIBAP website. For instance:
The first one is MusaLit, the online, searchable database on banana research publications:
Type in "Mycosphaerella fijiensis". Some of them are downloadable, but others can be ordered. (The SPC librarian also provides research documents upon request; they can also be obtained from the SPC biosecurity helpdesk). For the strobilurins, use "fungicides" as the search term.
INIBAP also has a publications site where information can be downloaded.
Integrating various options is the best way to go: resistant germplasm, looking at nutrition and perhaps alternating fungicides, and products to increase host plant vigour (some very interesting work is being done on some of these, like "vigor cal phos", in managing BLS). Sanitation options can form a strong part of a good management program, such as de-leafing and destroying infected leaves to reduce inoculum levels.
Punch and Tilt: are they no longer effective in Samoa?
Resistance to these two fungicides in banana develops very slowly over years of almost continuous use - as was the case in Central and South America. Their use in Samoa over the past 15 years will have been minimal compared to the large banana producing countries and restricted to relatively few plantations.
The apparent lack of efficacy by Punch and Tilt in Samoa is probably due to poor coverage or, more likely, low frequency of application.
For example, some growers feel that they do not have to spray during the first crop. That is true, they can usually get away with it the first year. But the build up of the disease on plants in that crop means that the ratoon plants are infected from a very early stage and, as they develop, the disease on them becomes very, very difficult to control. That is the time when most plantations collapse.
The secret of success is to keep the amount of disease in the crop (and hence source of spores) low from the beginning. The strobilurins are good against this disease, but are very prone to resistance. Manufacturers recommendations in most crops is that they should not be used more than two or three times per year and then in alternation with a different chemical group. Mancozeb is still an effective alternative for the dry season.
The problem with many banana clones with resistance to BLS is that they usually do not appeal to local tastes. AAA Cavendish is now the accepted taste in Samoa and to get a BLS resistant dessert banana with a similar taste will be very difficult, perhaps impossible. However, before Cavendish was brought to Samoa, local bananas with different tastes, such as those in the AAB Maia Maoli-Popoulu subgroup, must have satisfied local requirements. It may be worth experimenting with a few.
As for others:
'FHIA-23' - AAAA Honduran bred dessert banana that has 'tolerance' to BLS. Said to be twice as productive as Cavendish when no fungicides are applied to control BLS.
'PA-03-22' - Possibly an AAAB dessert banana bred in Brazil and derived from AAB 'Prata Ana', which is in the Pome subgroup as is Australia's 'Lady Finger'. Would be surprised if it has much resistance to BLS as it was bred for resistance against Fusarium wilt and yellow Sigatoka.
'Saba' - BLS resistant ABB/BBB cooking banana from Southeast Asia.
'Pisang Ceylan' - BLS resistant AAB dessert banana popular in India as 'Mysore'. This could be successful in Samoa, but most clones of 'Mysore' have Banana streak virus. This gives the clone its characteristic chlorotic appearance, turning to necrotic streaks in leaves at certain times of the year. The virus is spread by mealybugs.
'Pisang Lilin' - BLS resistant AA dessert banana popular in southeast Asia.
In addition, 'Yangambi Km 5' is a BLS resistant AAA dessert banana that could be trialled in Samoa and given the local taste test. 'Pisang Awak' is a BLS resistant ABB cooking banana that is very popular in Thailand and other Asian countries and may also have a niche in Samoa.
'Pisang Awak' and perhaps 'Mysore' were introduced into Samoa in the late 1980s when ACIAR/QDP, Australia was involved in BLS screening trials to determine if there was a clone that had potential in the South Pacific.