Concern over the increasing threat of early blight (Alternaria) plus more aggressive late blight in the UK should lead growers to return to Dithane (mancozeb) as their fungicide of choice in potatoes for blight management programmes this year.
"Alternaria has become much more of a problem in the UK, but it is always alongside late blight (Phytophthora infestans) and is rarely found in isolation. Growers will therefore need fungicides with dual activity on both these diseases. The protectant fungicide mancozeb is regarded as a key active against Alternaria on the Continent where this disease is a significant problem and the newly re-registered Dithane now has a label recommendation for early blight in potatoes in the UK. Curiously there is some view that the decline in the use of mancozeb as the mainstay active in blight programmes could have some impact on the reported increased risk of Alternaria, as most other blight fungicides, particularly the newer chemistry, have little or no activity on this disease," says Dr. David Stormonth, Technical Manager for Interfarm UK Ltd.
David points out that it makes no sense to apply a product for Alternaria alone, but does make sense to incorporate Dithane into programmes to keep both early and late blight under control, particularly if growing susceptible varieties such as Markies and Maris Piper. "You could consider mixing straight Dithane to other late blight products to increase the mancozeb concentration and assist in keeping Alternaria under control."
David advises growers to plan their requirements with their distributor to ensure a good and timely supply of Dithane. "Both fungicides are fully available to UK growers this year, as the manufacture and supply chain issues experienced last year have now been overcome. We anticipate that growers should not have any problems getting either product and so will be in a position to enjoy highly cost-effective multi-site protection against potato blight."
"Dithane NT and Dithane 945 have both been re-registered onto Annex III by UK’s CRD this year and have new MAPP numbers and re-freshed labels, supported by modern up-to-date field data for crops and diseases listed. Re-registration means that Dithane has had to prove that it is effective against both early and late potato blight and so is a strong independent affirmation of the products’ efficacy."
Dr. Stormonth reports that, following the re-registration of Dithane NT and Dithane 945, the dose rates for both products against potato blight will stay the same at 2kg/ha. "These two products are the only mancozeb products with a 2kgs/ha dose rate, which delivers 1500 gms a.i./ha, necessary for Alternaria activity. Spray intervals also remain unchanged at 7 days, with longer intervals in lower disease risk situations. The harvest interval remains unchanged at 7 days. The only change is the maximum number of applications of 8 per crop, which still gives plenty of opportunity to use Dithane throughout a full programme. At 2 kgs/ha and up to 8 possible applications per season this gives Dithane the highest mancozeb loading per season."
David also expects that Dithane 945 and Dithane NT to be more popular this year, as they are low cost but effective options, particularly when the weather is borderline for late blight infections. "Every opportunity to make cost savings without reducing efficacy should be grabbed with both hands, as production costs increase."
Both Dithane NT and 945 retain wheat on their labels and Dithane NT has some new crops on its label - bulb onions, shallots, carrots and parsnips. "Potato growers growing other root crops can benefit from Dithane NT’s multi-site protection against onion rust and carrot Alternaria."
