MORE WARNINGS about early blight control of alternaria species have been issued this week, with a big reduction on the use of mancozeb seen as crucial to the disease’s rise.

Frontier agronomist, Reuben Morris, agrees that alternaria blight is becoming more prevalent, but as well as the loss of mancozeb, he reckons that there are more susceptible varieties, such as Markies, being grown.

“Mancozeb gives reasonable suppression of alternaria blight but the cost of the re-registration and the subsequent price increase on farm has seen it being replaced in recent seasons in many spray programmes,” he says.

“This has resulted in an increase in alternaria appearing in mid-season, which has drawn us to the conclusion that most of the current late blight products on the market are not giving appropriate levels of control.”

The disease overwinters primarily as spores in soil and plant debris, with primary spores being produced in the spring, which are then rain splashed or wind dispersed to lower plant leaves. Spores can develop within two hours between 7°C and 35°C and irrigated crops are most at risk, with young actively growing crops are less susceptible.

“The yellowing caused by alternaria blight can be confused with magnesium deficiency. A diagnostic feature of alternaria blight is dark small spots on leaves. The spots often contain concentric rings.

“While the disease does not always appear to knock yield, it is very aggressive. Disease symptoms include rapid yellowing of foliage followed quickly by defoliation,” says Dr Morris, who adds that the solution is to start blight programmes early and base them on products that provide both phytophthora and alternaria blight control.

Also, John Edmonds, the European technical manager for Gowan Comércio, reports that in the ‘Euroblight’ ratings, Electis has a 2.5* efficacy rating for alternaria solani and alternaria alternata (in a 3* rating system).

He reckons that this blight fungicide, which contains zoxamide and mancozeb, is best for control of early blight as well as one of the best ratings for protectant activity against late blight and an acceptable rating for tuber blight.

“There has been a lot more concern about the increased threat of Alternaria, or early blight, in potatoes in the UK in the last two years, and this new disease has already been found in the popular varieties such as Markies, Saturna or Estima,” he points out.

Although Electis doesn’t have label approval for alternaria in the UK as yet, it does have full approval in some European countries, including France and the Netherlands, where this disease has been a significant problem for many years.