Towards a Decision Support System for Alternaria (early blight)

Recent articles, for example in the May issue of Potato Review, suggest that management strategies for early blight caused by Alternaria solani will be based increasingly on Decision Support Systems (DSS). So what is the state of play with such systems? In the US they have been developed over a period exceeding 45 years. They are based on the idea that you can stop the disease in its tracks, when the first spots can be seen on the oldest leaves and before these have generated sufficient spores to cause new infections. The development has gone through several stages, each with its own merits:

1 identifying the first points of infection

This requires intensive inspection and you need to be confident that you know what you are looking for. If you identify the disease, after it has had a chance to form fresh spores and new infections, you will be too late.

2 spore trapping

Checking the spore traps is time consuming and there is a risk of a wrong identification, especially if the operator is not familiar with similar Alternaria species.

3 modelling the development of the crop

The first appearance on the oldest leaves is often determined by the age of plants. This can be predicted though simple day degrees or more complex models for plant development. This system appears to have worked well in central states of the US.

4 modelling the progress of infection by the fungus

The most desirable of systems, because it will avoid the need for spraying when conditions are unfavourable. This requires the measurement of the conditions which are favourable to to infection and spread (relative humidity, temperature, rainfall and leaf wetness). Similar systems have been developed for late blight and the two have been integrated successfully in the US and appear to reflect Alternaria risk well.

Alternaria is a relative newcomer to the UK and therefore there has been little opportunity to evaluate systems in this climate. It is therefore not surprising that UAP have announced that they will use spore traps as a basis for recommendations. The presence or absence of spores is a common sense indicator. The reports that Forecast-Xtra, the late blight DSS, will include early blight as well, suggests that we may be moving in the direction of a full pathogen model.

Meanwhile there are a few practical guidelines:

• Closely watch the lower leaves of crops during and towards the end of the rapid growth stage and expansion of the primary stem

• We have little objective information on which varieties are most susceptible but lots of anecdotal information. Markies appears one of the worst varieties followed closely by Estima and Saturna. Other varieties to watch out for are Cultra, Hermes, Lady Rosetta, Maris Piper, Mimi and Russet Burbank.

• Make sure that you have symptoms checked out by a competent lab. Alternaria alternata causes similar symptoms but is less agressive. There are several other foliage diseases, which can complicate the picture especially late in the season, when the foliage is less tidy. Examples are grey mould (Botryotinia fuckeliana), white mould (Sclerotinia sclerotiorum) and Verticillium wilt (Verticillium dahliae)