June 30, 2005
Source: Farmers Weekly, 28 Jun 2005 [edited] <http://www.fwi.co.uk/Articles/2005/06/28/87993/Isolated+blight+outbreaks+rising.htm>

Isolated potato late blight outbreaks rising in England, Wales and Scotland

Recent thundery weather has lead to further development of potato late blight [PLB] in some areas, according to latest British Potato Council (BPC) reports. In the past week (w/e June 27) there have been a further 6 outbreaks confirmed in England and Wales, plus one more in Scotland, although overall blight incidence remains below 2004, experts said.

High humidity, high nighttime temperatures, and showers have given a flush of Smith Periods [see moderator comment below] in the Southwest, southern England, coastal Wales and parts of northern England, said Nick Bradshaw of ADAS.

"A feature of all the crop outbreaks is that they are described as 'scattered infections throughout the crop'. This strongly indicates the presence of a source of sprouting material somewhere in the vicinity of these outbreaks."

With wetter weather forecast, which will encourage the disease, growers should maintain robust spray programmes and maintain short intervals while crops are going through rapid haulm growth, he said [haulm is the stem or top of a cultivated plant].

Crops not sprayed sufficiently during rapid haulm growth are more likely to be infected later in the growing season when the risk increases, added the Scottish Agricultural College's Ruairidh Bain.

According to Bain, it is vital to remember that blight risk can be very localised. "What matters is the risk status where crops are and therefore access to good local information is important."

Visit the BPC's blight mapping website for latest regional blight reports.

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ProMED-mail
<promed@promedmail.org>

[Potato late blight [PLB], caused by _Phytophthora infestans_, is one of the most devastating plant diseases. Managing potato crops involves forecasting for PLB, which depends upon the probability of weather conditions that will favour PLB and assumes the existence somewhere in the area of a primary source of infection; the actual
level of inoculum present at any time is unknown. Many spraying decisions will be made in the light of -- and before the onset of -- deteriorating weather. The Smith Period is used to identify periods of high risk of disease spread, i.e., when the temperature and humidity favour blight. Smith Periods are defined as: 2 consecutive 24-hour periods in which the minimum temperature is 10 C or above and in each of which there are at least 11 hours with a relative humidity above 90 percent. In practise, the occurrence of a Smith Period provides a wake-up call.

Management of PLB has traditionally relied on copper, which is a broad-spectrum fungicide which protects against disease. It has been superseded by modern systemic fungicides, such as metalaxyl and furalaxyl. However, PLB overcame these plants because new strains (physiologic races) developed. New cultivars were developed that have "polygenic" or "field" resistance to the PLB pathogen. Combinations of several "minor" genes, none of which gives absolute resistance, are now used, and together they impede the rate of fungal development and enable the plant to tolerate infection.

Links:
<www.potato.org.uk/blight> <http://helios.bto.ed.ac.uk/bto/microbes/blight.htm>
<http://helios.bto.ed.ac.uk/bto/microbes/blight.htm> 
- Mod.DH]

[see also in the archive:
2004
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Potato late blight - Ireland: 1st report 200420040602.1499 Phytophthora sp., root and foot rot, tomato - Belgium 20040123.0266 2003
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Late blight, potato - Papua New Guinea 20030306.0554
2001
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Phytophthora infestans, potato late blight - Russia 20010620.1177 Phytophthora infestans, potato late blight - Russia 20010430.0840]