Blight

Blight is a common disease that affects potatoes and tomatoes. It is a serious problem which is more or less endemic in the UK. It has affected my potatoes every year recently, and needs thought and prompt action to avoid losing your crop. 

There are two kinds of blight, early and late blight. Both are fungal. Both are borne on the rain. Most years you are likely to get both. Late blight is the worse, being the disease that caused the Irish potato famine. If you mismanage it you can lose some or all of your crop and your garden can become a source of infection in future years. Early blight can be serious too, but is often mild. Again managing it well is the key.

The following sites contain detailed information about natural history,  identification and damage caused by the disease. The first part of the battle is to know your enemy! Read the following carefully…

Late Blight

• Potatoes Blight – Allotment.org (UK)
• What is Potato Blight? – Potato Council – home growers advice (pdf) (UK)

Late AND Early Blight

• Potato Diseases – Michigan State University (USA)

KEY POINTS in reducing loses due to Early or Late Blight

1. Grow blight resistant varieties.
2. Late blight mostly affects late harvesting potatoes (maincrop varieties), so plan to grow only first Earlies and Second Earlies. Second Earlies produce a smaller crop, but can be stored, so grow enough to replace your Maincrops. (I harvest mine the 1st week in September)
3. Make sure you dig out all of last year’s crop. Don’t become a source of next year’s infection.
4. Don’t compost old potato plant remains – bin them.
5. Be vigilant – check plants often.
6. Know how to identify Early and Late Blight symptoms. (Which one is the photo at the top showing?)
7. Warm wet weather makes infection more likely.
8. Keep foliage as dry as possible (I have heard of some people errecting polythese screens over their beds. Sounds very difficult!)
9. Only water at the base of the plants. Use tapwater, as fresh rainwater may be infected.
10. Earth up potatoes to prevent spores reaching tubers.
11. Late Blight spreads following a ‘Smith’s period’. Sign up with Blight Watch – a service that will alert you by email if there is a Smith’s Period in your postcode area, or if Blight has been identified in your area.
12. If a Smith’s period is forcast, spray all parts of potato and tomato plants to help prevent infection with Bordeux mixture (an organic fungicide, but its still not clever to put copper on your food!) or another approved fungicide.
13. Check the Potato Councils Blight Maps to see how many cases have been identified this year, and in which parts of the country.
14. If you see any signs of Blight, remove the affected leaves or stems and bin immediately – don’t compost. Then be extra vigilant.
15. Take hygeine seriously – wash hands and equipment if you have just handled infected material. Don’t transfer disease to healthy plants by touching them with dirty tools, hands or boots. Remember tomatoes can infect potatoes and vice-versa.
16. If Late blight spreads out of control, remove all top growth and bin it. Leave for 3 weeks then dig up tubers which should not be infected. You will have a reduced crop, but not the inevitable disaster if you leave it to spread unchecked.
17. If Early Blight is spreading through your crop uncontrollably, expect a 20% reduction in yeild.
18. Grow tomatoes under cover (greenhouse), not outdoors. Try to make sure that rain cannot get in and infect them. Water with tap water only. (If greenhouse tomatoes get infected blight can spread very fast in the warm moist conditions)
19. Some tomatoes appear slightly more resistant than others – cherry and bush varieties seem to succumb last.
20. If you are losing the fight, harvest entire trusses of green tomatoes and either try ripening them indoors (but be aware that many may develop signs of blight and be inedible), or use them green in chutney and cooking before the Blight spoils them (it won’t harm you).

All of the above are important points, and none can be ignored (although last year I managed without any spraying). However, it is really helpful to grow resistant varieties.

I have found that using the above factors I am quite confident about harvesting good potatoes each year. I have given up trying to grow tomatoes outside, and I am careful about letting rain into the greenhouse.