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NEW Beat the Blight
Potatoes and Tomatoes are two of the most popular crops for allotment growers and home gardeners and many of us wait in anticipation for the harvest year after year. This makes it even more heartbreaking when our crops are destroyed by the dreaded blight.
Potato blight is the worst disease for the potato and tomato grower. It can wipe out your plants overnight and then worse still it can infect the potatoes once in storage causing them to rot in the sack and eventually if not checked will ruin the whole lot.
What causes Blight?
Blight is caused by a fungus known as Phytophthora Infestans which commonly effects members of the potato family such as the tomato. When the weather becomes humid and wet (a normal British summer for us!) the fungus is ready to be dispersed by wind and rain. It only takes approximately 3 to four days of these conditions for the spores to be released into the atmosphere. Potatoes are normally the first to show the signs. The period of warm, wet weather necessary for the spores to become active is know as the "Smiths Period".
Smiths Period
Blight forecasting often based on the occurrence of "Smith periods". This is a 48 hour period in which the minimum temperature is 10°C or more and the relative humidity exceeds 90% for at least 11 hours during the first 24 hours and for at least 11 hours again during the final 24 hours. However, any period of warm, humid weather increases the risk of blight.
Spot the signs
The first thing you notice when blight attacks are brown speckles on the leaves or sections of the plants with brown and yelowish patches, these then turn black and a white bloom develops on the underside as the foliage dies. Spores produced by this bloom are washed down to the tubers under the soil causing dark spots on the skin and brown stains which spread throughout the flesh, this in turn changes the potato into a mushy, throughly smelly mass.
At the first signs of infection cut off the topgrowth and clear away any leaves that have fallen on the soil and destroy it(burning is best if you can) this will prevent the spores being washed down to the rest of the potoato crop. The potatoes will be ok in the ground for a couple of weeks but if you can try and get them our sooner. Sort your potatoes carefully, check for any signs of blight, any that look even slightly suspect get rid of them. If you store even one infected potato it will contaminate your whole crop.
NOTE: It is important to try and remove all your crop of potatoes from the soil as blight survives throughout the winter as spores in the potatoes left behind. This fungus then grows on the new shoots from these tubers the following year, these then produce more spores which are carried on the wind and infect your new crop when conditions are just right. Early crops have normally been harvested before serious attacks.
Prevention
The blight fungus is normally killed off by cold weather. The are a few chemical treatments available to treat this disease nowdays, the best form of treatment is to use Dithane 945 which works well but must be applied before the blight strikes. This obviously is not an option for the organic gardener so most of us have to face the fact that blight is never far away and if conditions are right it will attack. Other tips for prevention include making sure potatoes are kept well earthed up and try and water them from the bottom rather than spraying the folliage.
Probably one of the best ways to try and prevent it is to grow a blight resistant variety of potato/tomato, these are not totally immune but the crop is less likely to be serverly affected.
Good Potato varieties to try include:
Early - Orla, Karlena, Colleen, Premiere
Second Earlies - Spunta, Cosmos, Nadine, Milva
Maincrop - Stirling, Carla, Romano, Pentland Javlin, Verity, Sante, Sarpo Mira
Good Tomato varieties to try:
Legend(bush), Ferline F1 Hybrid(cordon), Fantasio F1 Hybrid(cordon) ideal for outdoor growing.
Useful Links
The Potato Council - The Potato Council has offered a blight incident reporting service to the British potato industry for the past 6 years. This information is collected on a voluntary basis by 300 blight scouts drawn from members of the industry who are routinely walking potato fields during the season.
RHS - Information on potato blight
SWCAA Allotment basics section on pest and diseases.