With the discovery of the fluazinam tolerant strain 37_A2 in UK potatoes last year, this will mean a rethink of blight control programmes, says Dr Reuben Morris, Crop Production Specialist for Frontier Agriculture. “In short term crops as in green top lifted there may well be a place for fluazinam but in crops intended for storage where tuber blight has a big impact, its use needs substituting. The use of fluazinam will become more limited overall this year, “ he says.
“As advisors we need to plan how we can effectively substitute for fluazinam and in my view the most important decision comes at the end of the programme where fluazinam was widely used in the past. I think the BASF fungicide Percos (ametoctradin and dimethomorph) fits very well as the last two sprays as it is compatible with diquat, whereas Raman Top (cyazofamid) use with diquat needs careful thought. Infinito is another option but its use can be restricted by end users.
Percos is one of the few fungicides that has tuber blight on its approved label. If you look carefully at blight fungicide labels, what do they actually say about tuber blight protection. None claim control of tuber blight, they give advice on reduction of tuber blight. The Percos label text says “It will reduce the incidence of tuber blight at harvest when used at recommended doses and spray intervals from tuber initiation to complete haulm desiccation. Ranman Top claims “tuber protection but warns growers not to rely upon it in place of end of season products approved with specific claims for control of tuber blight.” Infinito’s (fluopicolide and propamocarb) label states it will protect tubers from the risk of later blight infection after harvest and reduce tuber blight incidence when used as part of a full blight potato programme.”
“We are looking to replace fluazinam use in blight programmes, and in trials Percos has demonstrated tuber blight reduction equivalent to fluazinam, hence the label claim,” says Reuben. “Percos has tuber blight reduction on its approved label so this is my choice for the last sprays during desiccation with diquat.”
The loss of up to ten applications of fluazinam from blight programmes means careful planning is now required. “Percos contains the CAA fungicide dimethomorph and there are restrictions on how many you can use in a programme – up to 6 in total and no more than 3 consecutively. So we have to look backwards to plan our overall strategy. For example, if we used two Valbons (benthiavalicarb + mancozeb) and two Presidiums (zoxamide and dimethomorph) and finally two Percos applications during desiccation that would be the maximum CAA’s we are able to use,” he says.
“I would also consider the introduction of the new Corteva Agriscience fungicide Zorvec Enicade (oxathiapiprolin) from early rapid canopy to help replace the loss of fluazinam. This new product is supplied in a co-pack with Gachinko (amisulbrom) as a resistance strategy for oxathiapiprolin single-site active ingredient. “The tank-mix of Zorvec Eincade and Gachinko adds different forms of activity against zoospores,” he says, “This is important because trials have shown we must not ignore tuber blight control during rapid canopy as tubers are present from within a few weeks after emergence”
The UK blight population has changed a lot over recent years. The two dominant aggressive strains 13_A2, resistant to metalaxyl-m, and 6_A1 have now been joined with 37_A2 which is tolerant to fluazinam. “Blight control is a continuing challenge. We need to be very aware of resistance management. Sustainable resistance management advises use of sequences of at least four different effective modes of action. Percos provides an additional mode of action that is effective against tuber blight “ reminds Reuben.
Reuben Morris considers that “the cold wet winter this spring has led to delayed planting and could mean more blight during the establishment phase which is more difficult to control. At the other end of the crop’s life, if the crop has struggled to reach 100% ground cover, there may be a temptation to keep it in the ground for longer to bulk up. But this means more tuber blight risk, especially as conditions are more likely to be poor in late lifted crops.”
Paul Goddard BASF’s potato expert explains that Percos has two actives with different modes of action. This gives Percos built in resistance management making it a sound part of any blight control strategy, not the case with single active ingredient fungicides. Percos will control all known blight strains. “It is one of the most complete blight fungicides you can get, being able to hit blight at all stages of its life cycle,” says Paul. “I think growers will find appropriate places in the blight programme at the beginning, in the middle and at the end. But bear in mind it can only be used 4 times in a programme.”