Crop Disease Alert: Blackspot of Field Peas

In the last fortnight there have been a number of plant samples submitted and enquires regarding blackspot of field pea.

The disease has started to appear in commercial field pea crops, particularly those sown early for manuring.

Blackspot (as known as ascochyta blight) of field pea is the most common foliar disease of field pea in Australia and has traditionally been the main disease constraint to production. Back in the 1990's this disease was the main concern for producers and could result in significant yield losses if left unmanaged. The dry years of the early 2000's in combination with reduced field pea sowing has resulted in a greatly reduced incidence of the disease until this year.

The dry autumn this year in combination with a late autumn break and early sown field peas has been ideal for development of the disease. This has resulted in conditions where airborne ascospores are being released from old field pea stubble under cool, wet conditions which is favouring rapid infection and development. This disease is why the recommended sowing window for field pea in southern NSW is late May to late June, to avoid ascospore showers and disease development.

The disease starts as discrete circular brown lesions on lower leaves, these enlarge with time to kill off whole leaves. Stem infection will also occur with brown lesions appearing on the lower stem and crown of the plant, this will completely girdle the lower stem with time. Stem infection reduces nutrient and water flow within the plant and reduces yields.

The fungi that cause blackspot is a complex of three pathogens that can survive on the old stubble, on seed and as spores in soil ( a bit like brown leaf spot of lupin). So management of the disease relies on paddock rotation, separation from last year's stubble, fungicide treatment of field pea seed for sowing (with P pickle T) or foliar fungicides (products containing mancozeb or chlorothalonil). Foliar fungicide trials we conducted at the Institute in 1990's found that it was uneconomic to spray for blackspot (based on the price of fungicides at the time and the price of field pea). Having said that we would often achieve yield increases of up to 30% in some field pea varieties. Management of blackspot with foliar fungicides is most effective when the products are applied before the appearance of symptoms.

No current field pea varieties have resistance to the disease. Being a complex of three fungi it is difficult to breed for resistance. However, it is generally regarded that the more erect field pea types have better tolerance to the disease due to canopy architecture.

If field peas are to be sown for manuring diseases will become an issue, whether it be bacterial blight or blackspot. These crops are not bred to be sown in April and not be expected to develop disease. To maximise biomass the sowing time need only be a fortnight or 10 days earlier than ideal to see significant increases in biomass production. Early sown peas will develop disease early and suffer reduced biomass and yield as a result.

Further enquires. Contact Kurt Lindbeck, NSW I&I, Plant Pathologist P: 02 6938 1608