Top and bottom shots of affected leaves, note the whitish bloom of spores on some.
This is a serious plant disease which has been here for over 150 years. It caused the Great Irish Famine of 1845 to 1849 during which over a million people died and a similar number emigrated to escape the devastation. P. infestans affects the potato and tomato, both members of the Solonaceae Family; other members of the genera cause Phytophthora Root Rot in shrubs and trees.
The first symptoms of blight are a dark brown patch and yellowing of the
leaves, which turn black, then a white bloom
develops on the underside as the foliage dies. The
spores produced by this bloom are washed down to the
tubers causing dark spots on the skin and reddish-brown
stains through the flesh. It survives the winter as
mycelium and spores in tubers left behind at harvest. The fungus grows on shoots from these tubers the following year, and produces asexual spores that are carried by the wind
to infect the new crop during warm moist conditions.
Early crops are usually harvested before serious
attacks.
At the first signs of infection the topgrowth or haulms, can be cut off and destroyed to prevent the spores being washed down to the tubers - lift all leaf debris as well. The crop can be left for a few weeks before lifting, it will not develop any more - if possible , lift immediately. After harvesting the crop should be washed and carefully examined for signs of discolouring before storing and any suspect tubers set aside for immediate use - cut away any affected flesh and they can be eaten. (Commercial growers use a spray of Sulphuric acid or paraquat to kill the topgrowth 2 to 3 weeks before harvesting to reduce the risk of the haulms becoming infected and transferring spores to the crop as it is lifted.)
These are infected tubers. On the left the damage is slight so it can be pared away, the remaining part is not tainted and can be eaten.
Unfortunately the fungus is continually producing new strains which can attack resistant potato varieties. A new "mating type" of the fungus was recorded in 1978 which can introduce new pathogenic variants at a faster rate, so the battle to overcome the disease has become more intense.
Genetically Modified potatoes have been developed by including genes from a wild Mexican species, Solanum bulbocastanum, which has resistance to late blight. It was announced on 1st December 2006 that permission has been granted for the chemical company BASF to grow trial crops in Derbyshire and Cambridgeshire, England - they will begin in spring 2007.
Mild, wet summer weather and distraction from a rabbit attack elsewhere, meant this infection
progressed further than it should. The tops and leaf debris were immediately removed to protect
the tubers from falling spores.
Treatments
Prevention is better than cure so there are a number of measures which can be taken to avoid infection:-
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For chemical treatment:-
*Bordeaux Mixture is allowable under organic rules but too much is toxic to the plants:- (Two commercially available products are Murphy Traditional Copper or Vitax Bordeaux Mixture) |
To reduce the need to spray, forecasting the risk of attack has become very important. In Britain the "temperature-humidity rule" devised by Beaumont in 1947 is used. When 2 consecutive days of humidity over 75% and a temperature above 10 C occur, blight tends to develop 15 to 22 days later. A date is set for the high risk periods and it varies for different regions. These "Beaumont periods" are announced on the radio to give growers time to spray their crops before the disease sets in. This reduces the amount of chemicals used as spraying is only necessary if an attack is imminent.
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