When the chips are down Dealing with blight

VEG gardeners are busy buying potato tubers, sowing tomato seeds or ordering plants at the moment and we all want a successful crop. But the main blight on the horizon is Phytophthora infestans, a fungus-like disease with that name. Although we can’t avoid it, good garden hygiene and choice of varieties helps a lot. And that starts now.

Though always a headache for growers, late blight has developed ever-more devastating forms since the 1980s. Until then, there was only one strain of late blight, type A, so it could only reproduce asexually. But when a second strain, type 2, entered Europe as an invasive alien 30 years ago, sexual reproduction resulted in pernicious strains and continues to do so.

Conditions are right for these pathogens from midsummer onwards. The oospores need wet leaves, a temperature higher than 10C and a humidity greater than 90%. Timing depends on where you live and climate change has brought the date forward. My alarm goes off when I turn the calendar to August, but in lower-lying, warmer parts of Scotland it will be two or three weeks earlier.

If you see brown-black marks on leaves in June or possibly early July, this is early blight, Alternaria solani, a condition often caused by magnesium deficiency which does not seriously damage the plant or crop.

Symptoms of late blight are clear. Leaves, initially the tips, turn brown or black, with white undersides, then whole leaves wither. The shaws become a putrifying mess with tubers a rotting mush. This happens within a week or so.

But, as with so much in gardening, you can reduce the risk of blight through a watchful eye, good garden hygiene and choosing varieties with some disease resistance.

So buy certified seed potatoes. In summer check the potato foliage every day and at the first sign of trouble cut all the shaws to ground level and remove. Provided your compost heap is covered, composting them is perfectly safe.

Leave the tubers in the ground for a fortnight to allow the skins to harden and then use the crop quickly.

Tomato plants, especially those grown outdoors, are equally susceptible. The foliage shows similar symptoms. Harvest the fruits immediately and compost the plant in a covered compost bin or otherwise dispose of it.

Bush varieties may survive an attack. I’ve occasionally had an outbreak in the tunnel and saved the plants by immediately cutting off the infected leaves.

We have several other ways of preventing or reducing the damaging effects of late blight. As gardeners and plot-holders,

we must clean up our act because research at Bangor University has shown that our poor hygiene is responsible for much of the genetic diversity in the disease.

Immediately clear away and, preferably, compost all the vegetation after howking or digging a shaw. This removes an important breeding ground for disease.

Spores can persist in soil for up to four years, so practise crop rotation. The disease may also persist on tubers, even pea-sized ones in the compost heap.

And “volunteers” or “ground keepers”, the ones we missed when harvesting, can be carriers. Dig and bin them immediately. It’s totally irresponsible to treat their tubers as a “free bonus”. Your neighbours won’t thank you for your blighty present.

Keep tomato leaves dry when watering to prevent oospore germination and buy or build a tomato shelter if growing outdoors. My son introduced his wife to the joys of carpentry by urging her to construct one.

Finally breeders have developed some disease-resistant varieties. The Sarpo potato varieties are completely resistant, and new tomatoes, such as Mountain Magic and Honey Moon, with some resistance, are appearing every year.

Seed potato events

Glasgow Allotment Forum: Feb 24, 11-3pm, Reidale Neighbour Centre, 13 Whitevale Street, Dennistoun, Glasgow G21 1QW.

Dunblane Allotment Group: Mar 2, 1-3pm, Braeport Centre, Braeport, Dunblane.

Borders Organic Gardeners: Mar 3, 11-3pm, Springwood Park, Kelso.

Tomato plants, especially those grown outdoors, are susceptible to blight