By Iain Tolhurst

So many members have reported problems with potato blight (phytophera infestans). In GGI number Ten we gave some basic advice for home cultivation. Here Iain expands on this with guidance for home cultivation and field scale growers.

If growers were to follow the advice (in GGI 10) of lifting the crop early when rot was showing on some of the tubers they would find that the entire crop would probably rot in storage within ten days, as the blight spores would have been spread to all tubers at this stage. I have had over 25 years experience of organic potato growing having produced well over 1000 tons of crop in that time with little loss to this disease and have never used any copper based fungicides.

There has to be a systems approach to blight control starting with the correct choice of variety, some cultivars have a high resistance and these should not be placed adjacent to susceptible ones.

Blight resistant varieties include Orla, Sante, Cara, Provento, Cosmos, Marfona, Appell, Stirling, Teena, Torridon, Remarka and Romano. Some of these may be hard to track down but the Organic Gardening Catalogue has many of them.

Chitting is essential even with maincrop, to advance the crop and establish a decent yield prior to blight. Chitting means inducing the seed potatoes to develop small shoots before planting. To do this place the seed potatoes in trays rose end upwards (the rose end has all the ‘eyes’); put these trays in a well-lit but not over-sunny frost free room. After about 6 weeks small shoots will have grown.

Handle these chitted potatoes carefully and do not knock off the shoots. Start this process in about February, depending on when you intend to plant out. Plant out the tubers with the sprouts upwards of course, covering the fragile sprouts with a little fine soil or compost if you can.

Optimum fertility with respect to rotation and fertility building is important, as potatoes are very demanding of nutrient if a good yield is to be attained.

Monitoring for blight should be done daily as soon as ‘Mills periods’ are evident, this is the incidence of warm damp weather conditions and the presence of blight spores. Depending on the resistance degree of varieties, a decision has to be made as to whether it is prudent to mow or cut off the foliage or to leave it to die back naturally.

Some resistant varieties will often grow back again with healthy foliage as soon as the Mills period has passed. In some cases it can be safe to allow 50% infection of foliage before mowing off. If mowing/cutting is undertaken the foliage must be left on the ground to rot, removing it will encourage the introduction of blight spores to the crop.

The crop must not be lifted for at least three weeks post mowing/cutting to allow the blight spores to dissipate. The same rule applies if the foliage has been allowed to die down naturally making sure that the haulm has died completely prior to harvesting.

The disease survives over winter in infected potato tubers which remain in the soil or in discarded heaps so it’s important to remove all tubers when harvesting and do not compost potatoes or tomatoes.

This article appeared in Growing Green International magazine Num 11 (Summer 2003), p21.