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










15 thoughts on “Potato Blight and Compost Tea

  1. Deano Martin Post author

    I have amended the compost tea recipe. I was tired when I wrote this late last night, and typed the wrong figure for kelp and molasses, and missed off the rockdust. The figures in the text are now correct.

  2. Kevin Mascarenhas

    Hello Deano,

    Great work, thank you for sharing the recipe, I’m amazed at how quickly the plant tissue responds. Did you apply this tea early in the morning? What time was this? Thanks for the tip about not spray in the evening. Also, where in your thinking sits actively aerated composted that can multiple your bacterial tea by four times with a 24 hour additional brewing time?

    1. Deano Martin Post author

      Hi Kevin
      I applied about 6am, before the direct sunlight on those beds. It was overcast which was a bonus.
      I’m not sure that I understand the last question. Are you asking me whether this was Actively Aerated Compost Tea? If so, the answer is yes. I have a number of different sized compost tea brewers that I designed and made myself. This was made in a 50 – 80 liter size container. I have the capacity to run two of these, plus 2 x 200 liter containers at the same time, but I would never need that quantity of compost tea at the same time.

  3. Andy P

    Having never experienced blight before on potatoes I’m anything but an expert but I have always given them plenty of space and earthed them up well and they have always been in a draughty airy site which is mainly dry. How ever this year I have grown them in young horse manure ( 3 to 6 months old when planted) which has stayed damp. The bed is partly in shade. What I have noticed is that the area of the bed with the least sun has grown the best with the front of the bed getting sun being half the size of plant growth. Also in this sunny area one plant is suffering and many leaves have gone yellow (perhaps a magnesium deficiency) which may be a sign on blight.

    Since the disease is a fungal type disease it is worth working on the conditions that fungus likes. Warm damp places clearly need to be avoided. I would have thought the easiest way of avoiding blight is to site potatoes in a shady area which gets sun and allow the surface of the soil to dry out, which is pretty much where I normally plant, next to a hedge or fence.

    I also think planting as early as possible to avoid as much of the hot humid weather is worth while.

    This year has certainly seen a lot of humidity where I am as we have had lots of regular rain and the soil has stayed moist near the surface.

    I would have thought watering in the evening would be best since the warmth has gone out of the day and the moisture on the plants would help to cause a wind chill effect which may help to keep the conditions too cool for the fungus.

    1. Deano Martin Post author

      Hi Andy
      You raise some good points here. I think that providing a wider spacing would allow more airflow, and reduce humidity. I have some late maincrops in the other growing area which are planted at 3 ft spacings in mounds, and they look good. However they are in a soil that has recently had both lime and gypsum added, and with a lot lower organic matter content. All could be contributing to their success. They are in full sun
      The reason for not watering in the evenings is temperature. As it drops cool air holds less moisture than warm air, and the relative humidity rises.

  4. Patrick Whitefield

    Thanks for this, Deano. Very interesting. I’d like to encourage you to go with your idea of a more structured potato blight compost tea trial. That would be extremely useful. AACT is real leading edge stuff. In fact it’s revolutionary – treating disease by increasing the health of plant and system rather than killing the disease. I know this is the basis of organic growing but organics hasn’t got there yet in the case of fungus diseases. But we still don’t know nearly enough about how effective AACT is or how it works.

    By the way, what do you do with your blighted haulms?

    All the best, Patrick

    1. Deano Martin Post author

      Hi Patrick
      In the past I have composted them, or burnt them. At the moment I am flat out so they are going out to be recycled.
      In terms of a trial, it’s unlikely to be something that I would do. Not only is it much lower on my list of priorities than my grain trials, and the new system that I’m developing, but I also couldn’t let a control crop go to ruin without intervening. Unlike an academic researcher, this is my food. It’s not an economic thing, but I couldn’t buy food as nutrient dense as this anywhere to replace what I might lose. What I will be doing is thinking about what else could be done, and continuing to try new ideas out.
      Hope you are well

  5. Sue Rine

    Hi Deano, therehas been work done here in NZ at Lincoln University’s Biological Husbandry Unit (BHU) on the use of mesh covers to prevent infestation of tomato/ potato psyllid. An unexpected benefit has been a great reduction in blight.

    1. Deano Martin Post author

      Hi Sue
      That’s interesting. I wonder if it’s a shade thing (see Andy’s comment below), or whether the covers reduce splashback from rain or irrigation?
      If you have some links to share I’d be interested to follow them up

  6. Ute

    Deano, perhaps you could point the project leaders (Lucius Tamm and Bernhard Speiser) of the FiBL blight project BLIGHT – MOP to this blog. Who knows, they might be able to take this further.
    http://www.fibl.org/en/switzerland/projectdatabase/projectitem.html?tx_projectlist_pi1%5Bitem%5D=1&tx_projectlist_pi1%5Bstate%5D=detail&cHash=6346a102a26beab96e15e94f6421440e .
    There are also British contacts for the project, and a summary of the project and its results are here: http://research.ncl.ac.uk/nefg/blightmop/page.php?page=1
    Note the following:
    “Compost extracts: This method is not yet developed enough for practical applicability. It is not clear which compost feedstocks and methods of preparing extracts should be used, or how often and at what concentration the extracts should be used.
    Foliar sprays and microbial inocula: There has been success in some crops, but there is not conclusive demonstration of effectiveness against blight under field conditions.
    Microbial antagonists and plant extracts: Spraying antagonists and plant extracts was effective up to 70% in glasshouse trials and 45% in semi-field trials, but had low effect under field conditions. ”
    Perhaps they should have put all three together like you did.

  7. Andy P

    Having read up about blight it would appear that if you don’t want to use fungicide, which most of us wouldn’t, then there are a few key points to bare in mind.

    The “Phytophthora infestans” which cause blight can be predicted using one of several methods but is basically about monitoring the temperature and humidity. If the temp fails to fall below 10 deg C and maintains a humidity of greater than 75% for approx 48 hours (or if humidity is above 90% for 11 hours) then blight is likely to occur.

    Earthing up the plant stems is thought to slow down or impede the infestation from reaching the tubers.

    The tubers left in the ground from one season to the next are the main problem that creates inoculate for the next season so remove all tubers. Simple growing somewhere different wont necessarily help since the infestation will still be in the previous location and can spread via wind from the odd plant that grows in the previous location to the new location.

    The tubers stop growing if the plant loses 75% of its leaves so harvesting at that point will give less time for the blight to get to tubers and continue the cycle.

    Since the Phytophthora infestans require warmth and humidity I think it makes more sense to stop that situation from happening in the first place rather than finding remedies. Shade to some degree would appear to be key as is air flow but by keeping an eye on humidity you can predict the day you need to take action. Action on that day may mean a very good watering to lower the temperature sufficiently for long enough to stop the blight from forming.

    ….watering with a compost tea may be having an effect simply because you have managed to lower the temperature at a key time when blight would have formed rather than what is in your tea.

    1. Andy P

      I monitor my weather and store all the data and write a computer program to analyse the weather. I’ve just updated it to work out what days could promote blight, ie, 46 hours out of any 48 hours above 10 deg C where the humidity is above 75%. Only a rough guide.

      Testing all days this year I see that for:

      Possible blight day(s) 27,28 or 28,29

      Possible blight day(s) 10,11

      These were the only days where the conditions would have been right for blight.

      Technically the measurements should have been taken within the potato canopy rather than just the general weather conditions but it won’t be far out.

      If you only look at the temperature then most days in may and june would qualify and all days in july but if you then include humidity it narrows the possible time frames down to just a few days.

      Hope it is useful or interesting to someone 🙂

  8. Andy P

    The other thing about a compost tea, apart from a cooling effect, is the question of does it contain higher levels of copper, which are thought or known to inhibit the blight formation.

    In my thinking, when you need 2 days of favourable conditions it would be worth detecting the first day (temp and humidity being high enough) and water with your compost tea, which may cool, and include higher levels of copper (but not a high level) to break the 2 days of perfect conditions that are needed for blight to start.

    Combine this by siting the potatoes in a cooler spot with good air flow and making sure you remove all tubers each year or instantly dig them up when you see a missed tuber from last year start it’s growth, earth them up well and you should be well on the way to avoiding blight.

    Blight on a large commercial scale is hard to beat since you don’t have many choices in siting the plants plus difficulty to change the conditions or habitat when you detect favourable conditions for blight but on a small scale in a back garden you have far more control.

    Another question is did you have blight last year? My weather readings for last year suggested that there were no spells of 2 days where the conditions were right in my garden to promote blight….and I didn’t have blight.

    Of course, you don’t need to have the right conditions to start blight in order to get blight because blight may have started elsewhere and moved in on the wind or already have been there in the soil. Siting the potatoes next to a wind break that reduces the prevailing wind during may june and july would reduce wind brought in spores.

    Another thing to know is how long blight remains in the soil, and can it be killed by keeping the soil very dry for a short period. If so you could regularly dig and bring soil to the surface to dry out on your previous plot which had blight. Perhaps turning the top x number of inches of soil into a compost heap, mixing with manure and making sure the heap gets to a high temperature may break the cycle.

    Interesting subject, many variables.

    Fortunately blight is not a problem for me (yet) and therefore not worth me experimenting but I may well predict the start of a 2 day perfect condition cycle and water well and cover the plants to keep the temperature down for a few hours over night.

    1. Deano Martin Post author

      Hi Andy
      I get blight every year, without exception. My soil means that there is almost always more moisture close to the ground than the surrounding air, and the location and shelter of the growing area makes it difficult to promote more airflow around the plants. That siad I think that there are a number of measures worth trying.
      Firstly to space tuber far more widely apart, and treat them almost like single potato bags. By earthing these up high an keeping the canopies of each plant well separated, I should be able to improve airflow.
      Secondly ensure a full and balanced range of minerals. Potatoes originate from rocky upland terrain, which would have a very different texture and composition from my deep clay. The benefits of the copper sulphate/lime combination may just be the copper and lime feeding the plant, and not directly acting on the infection.
      Thirdly I agree about removing tubers, however that’s difficult to do when they come up in a new crop, without damaging the young plants. However I have a new rotation to move the potatoes into, and this should give me more opportunities to get them all out.
      Thanks for your research.

  9. Andy P

    While researching light and energy levels on plants it occurred to me that if plants don’t use the full light spectrum to photosynthesise then perhaps blight spores require a different frequency of light to reproduce. Filtering out certain parts of the spectrum could prevent blight without harming the plant.

    Does blight photosynthesise like any other plant? Is it effected by light.

    Would blocking the light for a couple of days stop blight, kill it, before it kills the plant. Then reintroduce the light so the plant can grow again?


Leave a Reply

Your email address will not be published. Required fields are marked *