Blight (Phytophthora infestans)

Plants/trees affected

Potatoes, Tomatoes
 

Symptoms

Dark/Brown spots appear on leaves often with a yellow surround.  A downy white growth, which is the spores, covers the underside of the leaf, the disease spreads rotting the whole leaf which will then curl and wither. The disease spreads rapidly affecting stems and eventually resulting in the collapse of the plant. Tubers will be affected as the fungus is washed off the leaves into the soil, creating brown sunken areas under which a red/brown rot takes hold. In this condition other fungi may invade the tuber leading to wet rot.

Causes

This disease is caused by a fungus and can be quite devastating as in the infamous Irish Potato famine of 1845-9. Spores are spread by wind and rain but also by infected tubers remaining in situ, which when they grow in subsequent years infect any new crops. The spores are prolific in warm moist conditions, usually occurring from July onwards and some strains can over winter in the ground or in potato sacks.

Prevention

Try to buy blight resistant crop varieties, planting them well spaced into the prevailing wind and not in shady areas. Use an organic mulch or a layer of hay/straw around the base of the plants to retain water and reduce the levels of tuber infection.  When watering is required, apply at the base rather than spraying the leaves. Destroying the plant tops before harvesting can also prevent the disease reaching the soil and the tubers.

Chemical sprays containing copper (Murphy Traditional Copper Fungicide) or mancozeb (Bio Dithane 945) can be applied at regular intervals to protect the plants before they become infected, but will be ineffective once the disease has taken hold.  Ensure you remove all tubers at the end of the growing season to avoid the possibility of harbouring the disease in the ground.

Treatment

• Remove and destroy as soon as possible any infected leaves and stems.  If the plant is more than 10% infected it is better to remove all of the stem and top foliage to avoid spreading the disease to other potato or tomato plants.
• Do not disturb the soil beneath infected and removed plants for up to 3 weeks, giving time for the spores to die and the skin on the tubers to thicken which will help prevent infection.
• Regularly check stored crops and remove any tubers showing signs of blight.