Saturday, 9 September 2006
In the last few days the weather has been much warmer and sunnier than it was through most of August, which is good news for the tomatoes (warmth is one of the things that triggers ripening) but has resulted in a significant increase of powdery mildew in my pea crops. To be honest, mildew is pretty inevitable if you grow peas late in the season in England, because it thrives on warm dry days and cool nights (especially if there's dew overnight) which is exactly what you get in an English September. I tried planting my late pea crop with a wider spacing than usual to improve ventilation around the plants, but they have still ended up coated with white powdery yukkiness. Alderman, a goddess among peas in every other respect, is particularly susceptible.
There are two basic types of mildew: powdery and downy. How can you tell the difference? Powdery mildew affects both sides of the leaf whereas downy mildew usually appears just on the undersides. Downy mildew is spread by sploshing water, whereas powdery mildew is airborne.
The one attacking my peas is the powdery type. And it's also having a go at my butternut squash.
Powdery mildew is a parasitic fungus (how charming) which spreads on wind-borne spores. The spores insert tubes into the leaf tissues and leach out nutrients. It can spread itself right through a crop in less than a week and causes a reduction in yields, and in extreme cases will kill the plants.
Normally my approach to mild outbreaks of things like this is to ignore them. In a balanced organic garden a lot of common minor diseases take care of themselves. In this instance though, I think the mildew is sufficiently bad that I have to treat it. A few weeks ago I had a very helpful tip in a comment from Bill and Libby, which I'm reproducing here:
"Looked up the specifics for milk- water treatment for powdery mildew: 1/4 c. milk plus 3/4 cup water, spray once a week. Once treatment is under way, a weaker solution will suffice. (When exposed to sunlight, a protein in milk produces oxygen radicals, which act as a natural fungicide.) Seems simple, anyhow.... from Herb Companion Magazine www.herbcompanion.com."
So that's what I've been trying today ... painting my mildewed peas with diluted milk! I'll report back on how they get on.
Treating mildewed pea plants with milk and water. I found it easier to paint it on with a brush as I've got a relatively small number of plants, but it can also be sprayed on.
Some research at Cornell University suggests that bicarbonate of soda works quite well too. Three teaspoons of baking soda in a gallon of water, sprayed on once a week. They don't specifically mention using it on peas, but it might be worth a try.
Because of the prevalence of mildew in the British climate I've been intending to make mildew resistance a factor in my pea breeding projects. I only know of one heritage variety, Ne Plus Ultra, which has resistance, but there are also quite a few modern varieties I can use to transfer the genes into my own stock. Powdery mildew resistance involves two recessive genes, officially known as er-1 and er-2 (catchy, eh?) and according to information from the Government of Alberta there is no such thing as partial resistance. It's either resistant or it isn't. But in practice it can't be quite that simple, because I'm seeing different levels of infection in different varieties grown together, with Alderman being the worst affected and Golden Sweet having hardly any, and the purple-podded types falling somewhere in between.
There are a few measures you can take to reduce the impact of powdery mildew. Growing resistant varieties is the obvious one. Keeping a good air circulation around the plants helps. And as water inhibits germination of the spores, a good soaking of the plants on a regular basis will help (not with downy mildew though ... it'll make that worse). Lush plant growth is especially vulnerable, so it's a good idea to avoid too much fertiliser, particularly nitrogen. When the crop is finished infected plants should be removed and not dug into the soil.