Late blight, Phytopthora infestans

In damp and warm conditions, this is a significant disease threat for tomatoes and potatoes. It’s not present in spring in temperate climates, although early blight may be in warmer climates. Here, I’m looking at late blight, a problem mainly in summer and early autumn.

Disease spores arrive in the air, during summers when night temperatures stay above 10°C/50°F for 48 hours, together with air humidity above 90% for all of that time.

These conditions allow blight spores to multiply rapidly. Leaves quickly turn translucent brown, followed by dark spots appearing on stems and tomatoes, and potato tubers. It’s not long before tomatoes and potatoes are rotting from the inside.

In cooler and especially dry summers, blight is unlikely to be a problem. It’s sometimes quoted as being present when it is not, and blight warnings are issued by manufacturers of fungicide when there is no need to worry.

Blight on potatoes

In warm and damp conditions, watch the health of your potato leaves. They should all be either green, or on older plants they will be turning somewhat yellow, which is fine. However, get busy when you see any sudden browning, and leaves going limp.

1. Cut off all infected foliage. If weather is wet all the time, best cut the plant to ground level. You can put this material on the compost heap.
2. As long as the cut stems are fresh green and not brown, leave them for two to three days so that blight spores on the soil die. Then pull your potatoes and put them somewhere to dry – perhaps in a box in the sun for a day or two – before placing in paper sacks.
3. If the cut stems show brown in the middle, that indicates blight spores have travelled down to the potatoes, and you will need to pull and eat them very quickly, if they are not already rotten.

• Check sacked potatoes a week later, to remove any rotten ones. Eat them quickly if some are rotting because that suggests the blight managed to enter many tubers, and they will not keep for long.

Blight on tomatoes

In temperate and damp climates, blight can be maddening for outdoor tomatoes, arriving just as they finally ripen in late summer. Salvage what you can, which may be little in damp weather.

If you spot blight damage on tomato leaves, best cut them off immediately, and likewise for infected fruits and trusses. However once the blight is in the plants’ sap, sufficiently to infect fruits and trusses, chances are you will not harvest any more healthy fruit.

Sometimes they look alright as unripe fruit, then start to brown and soften as they ripen. A further disappointment is that tomatoes ripened on blight infected plants do not taste sweet.

• Because dry leaves cannot host blight spores, water plants at root level only from early July. If you can keep leaves dry, all will be well.
• For outdoor tomatoes in cooler climates, grow early ripening varieties such as Sungold F1, and generally cherry rather than beef tomatoes. Cherry tomatoes ripen two to three weeks before beef tomatoes.
• It is safe to put any and all blighted material onto compost heaps, because blight spores need living tissue to survive. Therefore they die as soon as decomposition happens, which occurs even in cool heaps, just more slowly than in hot ones.
• For this reason there is no need to change the soil in a greenhouse or polytunnel where blight-infected tomatoes were growing. You can grow tomatoes there in the following year.

Mildew

The main two types are powdery and downy mildew.

1. Downy mildews are more destructive and a little less common. For example, the downy mildew of basil, Peronospora belbaharii, although rare in temperate climates, can kill basil within two to three weeks in warm conditions.
2. Powdery mildews are more common and occur on a wider range of plants. They are bright white, on the surface as well as undersides of leaves. Mostly they occur on older leaves, and plants continue to grow.

They also occur on leaves of plants grown out of season, which are therefore stressed. Examples are peas sown too late, and lamb’s lettuce sown too early. These are classic examples of disease happening as correction for mistakes!

Cucurbit downy mildew, Peronospora sparsa

This thrives in damp conditions and cooler nights. We suffer it sometimes at the end of summer, when suddenly you see yellow areas on cucumber leaves, which turn brown as the mildew takes hold. It can kill plants in two to three weeks, unless dry sunshine returns.

• When watering, keep the leaves of under cover cucumber plants dry.
• Outdoor cucumbers are at the mercy of prevailing weather conditions, and actually suffer more from powdery mildew.

Cucurbit powdery mildew, Podosphaera xanthii

On courgette/zucchini, squash and cucumber plants, from midsummer you notice white mildew on the lower leaves, with some pale yellowing of leaves. This is not as worrying as it looks, and does not prevent new growth. I have not known it to kill plants here.

You can cut off the mildewed leaves, wearing gloves for courgettes as precaution against their thorny stems. Plants will look nicer without necessarily growing any better.

• Sunlight and low humidity levels reduce the presence of mildew.
• Ensuring plants are well watered in hot conditions results is in less mildew.
• For respite rather than cure, mix one part milk to nine parts water and spray every two to three days. I do not find this worthwhile, but some gardeners do.

Lettuce downy mildew, Bremia lactucae

It’s not of great importance, but this is the most common disease of lettuce, on older leaves mainly. Especially in autumn when lettuce is not in season for leaf growth. It should not be significant in spring and summer, when your plants are growing in healthy soil.

Lettuce grown for hearts will always have some mildew on the outermost leaves, and this is not a worry. You can remove them if you wish. The important thing is how heart leaves stay mostly healthy, because they are younger.

• Frequent picking of outer leaves prevents mildew developing.
• Water in the morning not evening, so that leaves dry quickly.
• Expect some mildew on lettuce in autumn. Also in winter under cover, when you can allow two or three weeks between each watering of lettuce growing in soil.

Lettuce tipburn, Lactuca sativa

Hearting lettuce plants are bigger than those grown for regular leaf harvest. This means that they have more requirement for water, and you may see ‘tipburn’ (browning) on their leaves. It’s not a disease, but occurs from lack of water.

Onion downy mildew, Peronospora destructor

This mildew multiples in spells of wet weather in early summer, when your options are limited. I worry about it happening!

The first signs of infection are tips of leaves turning yellow, with some signs of white and then purple-grey mildew. I find that the mildew is difficult to spot on dark green onion leaves, often lining the undersides and out of view. Onions can be infected for a while before you realise.

During rainy weather, from late spring through to midsummer, watch your onion leaves closely. If you see this mildew and it’s still raining, you may not enjoy a good harvest of onion bulbs or of salad onions.

Removing leaves with mildew can reduce its spread a little, but if the weather stays done it soon returns. Infected plants are best pulled sooner rather than later, and the bulbs of infected onion plants will not store as long as normal.

• If your climate often has wet conditions in early summer, best grow resistant varieties such as Hylander and Santero, both F1.
• Allow more space between onion plantings for better air circulation; however in favourable conditions, the mildew spreads anyway.
• It’s worthwhile not to grow onions from sets, which risk carrying mildew into the onion plants.
• Overwintered plants for onion bulbs in early summer may mature before mildew is significant. On the other hand, their leaves in winter give the mildew a host to live on, unless winter is frosty.

Pea powdery mildew, Erisiphe pisi

Powdery mildew on pea leaves is totally normal from midsummer, and can be much reduced by sowing early in spring. You are then harvesting peas earlier, and finish most of the harvest before the mildew is too developed.

In contrast, the sowings of late spring will be ripening after midsummer, when mildew is more likely, and the result is peas not being sweet. Mildew prevents leaves from photosynthesising to produce sugars.

The lower leaves of plants are the first to show signs of mildew in early summer, especially in dry weather.

• One remedy, therefore, is to keep plants well watered.
• There is a mildew resistant variety of pea called Terrain, bred for late sowing. You can sow it even in early summer, for harvests in autumn.

See this video on YouTube for more on powdery mildew:

• Powdery mildew, why it happens, mostly not a worry

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