Tomato Blight, & The Related Potato Blight, Can Destroy Crops in Days But Can Be Controlled In Certain Cases. This Article Explains More About Tomato Blight, How To Prevent Blight and How to Control Blight.

Tomato blight
The start of tomato blight.

Tomato blight is caused by a fungi-like organism, Phytophora infestans, and can destroy crops in a few days. It is common in the UK in late summer and should not be confused with early blight which is common in the USA but extremely rare in the UK Early blight is caused by a different organism, and is a form of Altenaria.

Late Blight occurs on both tomatoes and potatoes when certain weather conditions allow it to sporulate, spread and geminate on new plants. It is not a problem when the temperature is under 10C. The conditions it needs to grow and spread have been extensively researched  by the James Hutton Insitute (funded by AHDB) and are known as Hutton Criteria. Hutton Periods are notified  to farmers and growers via the Syngenta website.

Tomato Blight Conditions

For blight to become a problem there needs to be cool moist conditions. And this makes it a problem in the UK where the prevailing Atlantic winds are moist and rain a common feature. Rain often brings cooler weather and hence we get cool moist “Blight” conditions.  In summer, when the weather has been as it was in 2021, wet and cool, blight is noticeably much worse.

The specific conditions needed to trigger a Hutton Period warning for Blight are temperatures above 10C and relative humidity above 90% for at least 6 hours on two consecutive days.

Spread is more likely as the temperatures warm above 10C and in my experience worse when between 18-25C. It should be noted that the main way in which blight is spread is an airborne spore. In this way it can be spread miles on a gentle breeze.

However, even when spread on the breeze, it will not be problematic unless the Hutton conditions trigger germination. Blight surrounds us a lot of the time but only devastates crops when conditions suit it.


Where Did Tomato Blight Originate?

Some report tomato blight as originating in the Andes which is where potatoes originated. But recent research by Niklaus J. Grünwald and Wilbert G. Flier, indicates that a more likely origin is Mexico.

Where it actually came from years ago is now an academic argument. It is now here and has been for many centuries. And it is the same blight that caused the Irish Potato Famine.

What Does Potato Blight Look Like?

Blight starts by causing chlorosis of the plant and this is very quickly by brown lesions on leaves and stems. In some cases a fine white hyphae like structure can be seen around the lesions, especially on the underside of leaves.

The name Phytophora is indicative of the disease on that in Greek phyto means plant and the remainder of the word “phthora” is Greek for decay, ruin, or perish. Infestans is pretty clear .. infested, infestation. The name was first suggested in 1876 by the German mycologist Heinrich Anton de Bary. It certainly describes the disease well in my view.

What is Tomato Blight?

I’ve described blight as fungi-like. That’s close enough for us to understand it. But technically (here’s the science bit) it is an oomycete or water mould. And this is evident by the watery look of the disease in certain conditions. Especially when fruit or tubers are infested.

How To Avoid Tomato Blight

Tomato blight spores are wind borne and, as far as I know, there’s no location in the UK where they are unlikely to exist. So the way to avoid the disease is to avoid the conditions that allow the spores to germinate. But unfortunately we can’t change the weather, so this is not a simple task.

However, when I grew tomatoes commercially we never had a problem with tomato blight in our greenhouses. And we didn’t spray fungicides.

A bouyant atmosphere deters blight

The simple way we avoided it was to keep the humidity below the blight threshold. In a greenhouse, where it tends to be warm, its relatively simple too control humidity and its doesn’t require special equipment. Growers always say to aim for a “buoyant” atmosphere in the glasshouse. Bouyant is hard to describe, you know it when you have it. But let me explain what it isn’t. It’s not a muggy atmosphere. If the atmosphere feels “close” or humid it’s not a good sign. And that’s why growing toms and cues in the same house is fraught with difficulty, If its too dry the cues don’t like it and yields are too low to be economic, and if its too humid you get blight on the toms!

In warm weather we allowed temperatures to rise but also gave the house plenty of ventilation. Good ventilation was the key.

Blight Resistant Tomato Varieties

No variety is blight free but some are far more tolerant than others. It’s the same with potatoes. Some earlies are prone to blight, but if lifted early are out of the ground before blight strikes.

Just look for varieties listed as blight resistant but don’t hold your breath, all toms can get blight if Hutton conditions exist for too long.

I’ve not mentioned the names of resistant varieties as the disease mutates frequently and what resisted it previously isnt always resistant forever. It’s why I have reservations about resistance claims.

Cultural Control of Blight

However, on tomatoes the main control methods are to use resistant varieties, and to ensure a buoyant atmosphere in greenhouses. I found that where I grow outdoor tomatoes I can sometimes avoid it by planting as early as possible (easier in the south) and putting a cover over the crop to lower humidity levels once Hutton periods start to trigger. It helps a bit.

If you can grow the crop against a south facing wall the heat from solar gain often reduces humidity in this microclimate. That can be enough to stop the spores germinating. It’s a marginal change but often enough.

If you see signs of blight you can remove a few infected leaves. But the chances are you are only going to delay the onslaught by a day or two. You are very unlikely to stop it. By this time the infection is in the plant. It’s like trying to cure the common cold by blowing your nose.

In potatoes farmers will often remove the haulm once they have a decent crop and fear blight.. They don’t want to harvest too early as they want the skins to set (harden and thicken a bit). They remove it by “topping” with a machine or by using a chemical desiccant. The reason they do this is to prevent any plants developing the disease and dropping spores on the soil. If they get washed down to the tubers and infect them they can cause rot in storage.

However, the spores don’t last long in the soil. And, in my view don’t survive composting (though many gardening groups say to destroy the diseased plants by burying very deep or going to landfill).  The real threat are the plants that survive the winter as they may carry dormant spores. The problem is especially bad with potatoes as some tubers survive as “ground keepers” and become weeds in the next crop. These can carry blight and be the source next year.

Farmers and growers have fungicides they can legally use. Gardeners have none and are at more risk.

Some gardeners advocate Bordeaux Mixture or a copper sulphate wash as a cure or preventative treatment for tomato and potato blight. True it can work but copper sulphate is highly toxic and can damage the soil and soil fauna as well as being dangerous to human health. Bordeaux Mixture is not permitted to be sold in the UK.

Spraying milk on the crop leaves is sometimes advocated as a cure for blight. But I can find no research to confirm it works.

Relevant Research

Tag: Tomato Blight

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2 thoughts on “Tomato Blight: Control and Prevention of Late Blight In UK Gardens & Allotments

  1. Christine Housley says:

    You don’t mention that it’s unwise to grow either host crop in the same spot for 3 years. This is what the RHS advocate as ordinary growers are unlikely to have the facilities to have removed the haulm earlier, therefore spores are likely to be now in the soil. I’ve avoided blight where I live on Anglesey, I don’t know if others have been susceptible to it here, but I grow in a peculiarly cool spot.

    1. Stefan Drew says:

      I’m not sure why the RHS suggest this as the spores don’t survive long in the soil. As for the host crops, it is relatively unimportant as blight is airborne and is everywhere in late summer ..just waiting for the right conditions. So susceptible plants or not make little difference

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