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Potato Blight and Compost Tea

I hadn’t intended writing a post on potato blight and compost tea to end my posting drought, but I had the opportunity to record the effect of using compost tea on blighted potatoes, and decided to run with it. Please note that if you’ve found this blog searching for remedies or preventative measures to combat potato blight,  I’m not claiming to have found a cure.

Potato Blight Conditions

Periods where potato blight is likely to occur are called ‘Smith Periods’. There’s a good explanation of Smith Periods on the Blightwatch site. I’ve been a bit busy and haven’t been checking the weather forecast, nor have I notified Blightwatch about a change of email address, so was horrified to find that two complete beds of potatoes were showing signs of potato blight. The two beds are shown in the picture below, after treatment.

potatoes in beds

Two beds of potatoes

The bed on the right is a double row of Charlotte potatoes. A lovely second early, salad potato, which should be ready to harvest soon. The bed on the left contains a single row of Lady Balfour potatoes, an early maincrop potato which I grew for the first time last year. Please note that these were taken after all of the treatments that I undertook.

Normally I spray my potatoes with compost tea before any likely Smith periods. I think that it helps to delay the onset of potato blight. I avoid spraying in the evenings, as the nights normally have higher humidity levels, and I want to avoid wet leaves at night. As I mentioned earlier, I’ve been busy with my grain crops, and it completely slipped my mind. The first thing that I normally do once I have potato blight is to cut off all affected foliage. In the picture below you can see how badly the right hand row had been blighted by the amount of leaves that I cut off. Yesterday I started a compost tea brew before defoliating so that it would be ready to spray by the following morning. (Recipe details later).

tTreatment for potato blight

Potatoes after trimming

Note that some of the top leaves had lots of small blight spots, but I wanted to leave a few leaves on each plant. After removing all of the infected foliage the potatoes didn’t look too bad, so I took off the clothes that I had been wearing whilst handling the infected foliage, along with the secateurs, and put them to one side to use for that purpose only, to avoid spreading the potato blight to a third bed in another area, and to the tomatoes and peppers in my polytunnel.

Applying Compost Tea

This morning when I checked the plants more leaves were showing large blotches of blight. I had hoped that the extra airflow may have checked the spread of the disease, but that hadn’t been the case. I then applied compost tea as a foliar spray, trying to wet the undersides of the leaves too. The compost tea was applied at a dilution of 1.9 with water, or 10%. 1/2 liter in a 5 liter sprayer. I have a larger backpack sprayer, but it wasn’t needed for this quantity of crop. 20 liters was enough to treat both rows.

Compost Tea Recipe

I used a simple bacterial compost tea recipe

50 liters of rainwater

500 cl of liquid kelp

500cl of molasses

2 liters of urine (approx)

500 cl of vermicompost

500cl of my best cold compost

500cl (approx)of finely sieved rockdust (basalt)

I forgot that I have made some fish hydrolysate and could have added that too.

Further Treatment

After the compost tea had dried I decided to spray with a kelp and seaweed foliar spray. I can’t really tell you why, but it felt right. My brain rationalised it by telling me that it was food for the plant to help it fight the infection, because the ingredients for the compost tea brew would largely have been incorporated into bacterial bodies. I don’t think that’s strictly true, and perhaps I just wanted to ‘do something’ extra to help.

The Results

A few hours after applying the second spray it rained heavily. So when I went back to check on the potatoes later in the afternoon I was expecting things to have deteriorated. It was immediately obvious from a distance that there hadn’t been an increase in blighted leaves, and in fact the bulk of the foliage looked healthier than it had this morning. A closer inspection of the leaves confirmed that. The picture below is of a leaf that had exhibited  patches of dark brown blight. You can see that the blighted tissue appears to be being eaten away from the center outwards. Where intact areas of blight remain the colour is a lighter brown, and areas of leaf tissue where blight infection had just begun has lightened up.

leaf treated for potato blight

the effect of using compost tea on blight


The picture below shows one of the upper leaves that had been covered in little dark spots of potato blight. Again the blight seems to be lighter in colour, and to be being eaten away.

blight on potato leaf

a leaf showing blight spots

The next picture shows another leaf with the spots almost gone.

potato blight on leaf

More blight spots

Further Actions

I’m going to give the potatoes another spray with compost tea tomorrow, after removing any leaves still showing signs of potato blight infection. I may also remove some more of the leaves from the Lady Balfour row, to give a better flow of air through the plants.


It is impossible to say with any certaintity how much of a role the compost tea played in the checking of this potato blight infection. The potatoes may have fought the infection off themselves. The improved airflow and reduced humidity created by a drastic pruning of the leaves may have been responsible, or contributed. The additional spray of kelp and molasses may have been the main component, which is more simple and quicker than starting a compost tea brew. However I’ve seen compost tea prevent blackspot on new rose leaves after spraying, and my potatoes are normally pretty late in getting blight when I am spraying them regularly with compost tea. It certainly seems like there is scope for a more structured potato blight compost tea trial.


All of the best










Building A DIY Warming Cabinet.

I’ve felt the need for a warming cabinet a few times in the last year or so. A late batch of cider failed to get started in the cold weather, and a warming cabinet, or warming pad, would have stopped the cider from ‘hanging’ and eventually being spoilt. Last Autumn I wanted to experiment with making biofertilisers, but many of them need some warmth to ferment properly. Again, a warming cabinet would have allowed me to experiment with the biofertilser. Finally, I have just bought some mushroom spawn, to grow edible mushrooms. They too need some warmth, so I finally got a grip of myself, and made one.

A quick disclaimer here. I didn’t bother looking to see how other people have made their own warming cabinets, I just knocked this one up. Please don’t copy this unless you can’t find anything better.

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