tomatoes with blight

How To Prevent Blight In Potatoes And Tomatoes: What Is Blight And How Does It Affect Crops?

Blight in tomatoes and potatoes, commonly known as late blight is a nightmare for most farmers of these crops. It can wreak devastating damages to plantations and farmlands. Its catastrophic effect is so great that it was singled out as the culprit that instigated the famous Irish potato famine in the middle of the nineteenth century.

Although, there are early and late blight conditions, late blight is the most common form of blight infection. Late blight is a disease caused by the pseudo-fungus oomycete pathogen, Phytophthora infestans, that infects and destroys the stems, tubers, leaves and fruits of tomato and potato plants causing decay. It occurs mostly in wet conditions.

Late blight in potatoes defoliates the crops. However, if the disease arrives after the tubers are formed and ripe for harvest, little to nothing is lost.

If the tubers are infected, they develop some reddish-brown granular markings on the flesh and brown discolouration on the surface. The tubers may seem solid but are bound to develop soft rot. This soft rot is caused and quickened by an invasion of bacteria that can cause tissue breakdown.

In tomatoes, dark, water-soaked lesions are formed on leaves with the lesions gradually growing from the leaves/margins of the plant towards the centre.

leaf with blight
blight infected potatoes

Lesions then develop into brown, dark patches which becomes brittle as soon as they dry out. These lesions also develop in the stems and fruits of the crop.

The causative organism, a fungus which belongs to the group Oomycetes is highly aggressive. It can affect all plant parts and can potentially cause quick diebacks and deaths. This organism spreads its infection through sporangia and zoospores.

Sporangia, known in the singular as sporangium, is mostly formed on infected foliage and then dispersed through water and wind media.

A legion of zoospores can be formed and enveloped within a sporangium. They can then swim in both films of water in the soil and on the plant's leaves in a bid to infect the plant's parts.

Blight in potatoes are potentially caused by diseased potatoes dumped on the farm or infected seeds, and unharvested potatoes abandoned to rot in the soil all of which have the potential to grow the following season.

How to Prevent Blight in Tomatoes and Potatoes

Blight can indeed cause serious worries for potatoes and tomatoes farmers. The ease in which the disease can spread is more of a concern to all growers.

However, if you take some precautions and put in place some carefully thought-out measures, you can groom a blight-free farmland.


Plant Resistant Crop Varieties

This is perhaps the most effective way of preventing a blight endemic. However, you must know that these crops are not totally immune and hence, not fail-safe.

Examples of resistant potatoes include Elba, Sarpo Mira, Orla, Colleen, Golden Wonder, Kerr’s Pink, Record and Defender. Other varieties like Allegany, Rosa and Sebago have also been shown to possess some resistance.

For tomatoes, there is a higher risk of the disease being spread around, even amongst resistant varieties. Some of the blight-resistant tomato varieties include 'Pruden's Purple', 'Matt's Wild Cherry' and 'Mr. Stripey'.

tomato farm not infected with tomato blight
potato farm without infection of blight

Keep the Disease Out of Your Garden

This might seem like the very logical thing to do, yet, it isn't as straightforward as it seems. Keeping the disease out of your garden means you must be careful to eradicate every potential sign or cause of infection.

If you've had a blight problem in or around your farm during the growing season, you should endeavour not to save potato tubers or tomato seeds for replanting the next growing season. However, they can be consumed as blight infections do not affect humans.

If there's a report of blight infections in your area during the rainy summer, spray some fungicides on your crop to help prevent late blight infection.

Choose the Right Site and Soil Conditions

You will be doing yourself no justice if your land prepared for cultivation is not well aerated. In an unaerated land, the leaves of your crops can potentially retain humidity, a condition under which both early and late blight conditions thrive.

The following tips could help:

  • Do not plant close to the woods.
  • Avoid heavy soils when cultivating potatoes. Sandy soils are a good alternative. There is a lower risk of blight spread in sandy acid soils than in other soil types.
  • Ensure that your farm area is level to avoid water accumulating or ponding.

Proper Sanitation

Asides planting resistant varieties, this is one of the most important preventive measures against blight in potatoes and tomatoes. Potato culls and tomato seeds are usually the main culprits of blight infections. Infected parts need to be burned, buried or composted before planting in the spring.

When buying potato culls, make sure you bury them deep in the soil and cover them with soil or plastic to block out any light that might help them photosynthesize and grow.

When burning, pile them up in thin layers over combustible materials. You can also expose the chopped culls to the extreme freezing temperatures of fall.

tomato plant infected with blight

In general, if you prefer this last option, it is recommended that the field on which you expose the chopped cells to freezing temperatures should not be used for growing potatoes or tomatoes the following year.


Mechanical harvesters are notorious for promoting late blight infection. While moving through the soil in potatoes, they spread the spores left lying in the ground to tubers that are not at any risk of getting in contact with the infected spores.

Thus, it is a good idea to only harvest in dry conditions. This is because infected spores thrive best in wet conditions. In fact, spores only affect potato tubers when wet.

If foliage infected with blight dies naturally, wait for at least fourteen days before harvesting. This is to allow room for the spores to die.

In general, you should take the following steps to help prevent both early and late blights

  • Make use of only disease-free seeds and tubers
  • Practice crop rotation by rotating either potatoes or tomatoes with another crop not affected by blight
  • Mulch under plants
  • Ensure proper spacing between plants to limit the rate of infection spread.
blight infected leaves
potatoe afflicted with blight

Blight Treatment in Potatoes and Tomatoes

If blight has sunk its teeth into your tomato or potato crops, all hope isn't lost yet. It is time to take treatment measures. Blight treatment in potatoes and tomatoes isn't as hard as you may think. With good blight fungicides, you can limit the spread of the infection.

Blight Treatment for Tomatoes

Blight treatment for tomatoes and potatoes are very similar – Infected leaves should be removed and burnt.

Spray the infected tomato plants with Demildex® copper oxychloride or other copper-based fungicides. You should continue spraying till the plant's leaves are dripping wet. For best results, apply spray when it is cloudy or early in the morning. You should avoid applying the fungicide at noon when it is hot outside. This is because applying fungicides under sweltering weather conditions can inadvertently burn the leaves.

tomato farm

For powdery copper fungicides, apply a thin layer of fungicide powder on both top and bottom of the leaves using pressure duster. You should reapply the fungicides every three to ten days until all blight symptoms vanish and the disease is cured.

You can also spray the infected tomato plants with baking soda spray. These sorts of sprays are good for killing fungal organisms and do a brilliant job of treating blight. They are also friendlier than copper fungicides.

Blight Treatment for Potatoes

If few leaves are affected by blight, you can just remove and dispose of the infected leaves to limit the spread. Otherwise, if you have a more severe infection, you might have to cut off all affected foliage and stem and either burn or compost them.

Removing the foliage from the infected plant helps prevent the disease from spreading and sinking into the tubers, as long as they are well converted with the soil.

Then, leave the crop for a while – about two weeks – to let the blight spores on the surface die and the potatoes develop a much thicker skin.

pile of potatoes

You may also spray your crops using copper fungicides, the most notable of which is copper oxychloride. Demildex® Copper oxychloride - a good blight fungicide - known in most quarters as dicopper chloride trihydroxide should be applied every 10-14 days following heavy rainfall or when the infection is rapidly increasing across your farm area. African Pegmatite can supply your needs.

If possible, it is recommended that you apply the fungicide such that there are 12 hours of dry weather following application.

In general, copper fungicides are applied at a rate of 1 to 3 teaspoons per gallon of water. Although copper fungicides are not harmful to bees, they shouldn't be applied when bees are actively foraging on plants.

You should never over-apply these fungicides, as an excessive concentration in the soil can have damaging environmental impact as well as have a counterproductive effect on the crop.

copper oxychloride demildex used to treat black spots on roses