tomatoes with blight

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

Put simply, blight can be fatal to a potato or tomato crop. Through a combination of farming best practices, great care and treatment with a systemic fungicide such as Demildex, the problem of blight can be a thing of the past. Copper oxychloride Demildex is a highly effective fungicide, useful for treating potato and tomato blights amongst a wide array of other fungal plant diseases.

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 identified as one of the major contributory factors leading to the 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.

Early Blight

Early blight is caused by Alternaria solani, a fungus. Overwintering in nightshade plants, conidia are produced asexually in the spring which are transmitted via wind, water or rain to uninfected plants. The weather conditions in the springtime are ideal for transmission: moist and not too warm. The fungus penetrates the plant through stomata or via direct injection, whereupon the fungus can grow, producing lesions. In potatoes, a yield reduction is typically experienced. The symptoms are lesions on stems and concentric circles on leaves. In the case of tomatoes, infection is through openings caused by growing cracks or by those created by insects, in addition to direct injection through stems. Concentric circles are again a feature on leaves and defoliation can occur.

leaf with blight

Defoliation can lead to a degree of sun scalding to the fruit, which will lead to spoilage and further cracking and proliferation of the fungus. In all cases, the fungus can produce more conidia which is easily spread. A. solani also has similar and profound effects on other plants in the family Solanaceae, including peppers and aubergines. A. solani can additionally cause ‘dropping off’, where seedlings and seeds are weakened before or after germination.

blight infected potatoes

Late Blight

Late blight is by far more destructive to a potato or tomato crop than early blight. Herein, strategies to manage the transmission and proliferation of late blight will be discussed.

The organism responsible for late blight is oomycete Phytophthora infestans. An oomycete is a fungus-like eukaryotic microorganism, and is treated in a similar way to as if it were a fungus.

Oomycetes are highly aggressive. They 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. The organism strongly favours moist and cool environments, with the production of zoospores typically occurring at temperatures below 15 °C - i.e. typically in the springtime. 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. Lesion growths typically are most efficient in the summer months, when temperatures exceed 20 °C.

Late blight in potatoes will cause the crop to defoliate. 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, often appearing ‘corky’ throughout the flesh. It should be noted that there are little to no known problems of consuming lightly blighted potatoes - the major problem lies with failed and rotted crops.

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

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 potato and tomato farmers, as a blighted crop is an unsellable one. The ease in which the disease can spread is more of a concern to all growers. A thorough crop management programme with the ability to take appropriate action is key to a successful harvest year after year.


Plant Resistant Crop Varieties

The planting of resistant varieties of potato and tomato plants is, naturally, the most viable method of avoiding blight. No plant carries 100% resistance, and so good crop management is still required.

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'.

It is noteworthy that some of the most popular varieties (particularly amongst potatoes, where ‘old’ varieties such as King Edward) are not resistant to blight, yet often account for the largest crops due to their commercial attractiveness. It is therefore crucial to maintain growing best practices and to take action if necessary.

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

Keep the Disease Out of the Growing Area

If there has been a blight problem in or around a growing field during the growing season, potato tubers (‘seed potatoes’) and tomato seeds should not be retained for planting ahead of the next growing season. However, they can be consumed as blight infections do not affect humans.

Because of how the disease is transmitted, reports of blight infections in the area during the rainy summer, the spraying of some fungicides on the crop to help prevent late blight infection is encouraged.

Choose the Right Site and Soil Conditions

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

  • 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 the growing area is level to avoid excessive water accumulation or pooling.

Proper Sanitation

Asides from planting resistant varieties, proper sanitation and maintenance of the growing area is one of the most important preventive measures against blight in potatoes and tomatoes, as potato hulls (including ‘seed potatoes’ from the previous growing season) and tomato seeds are usually the main culprits of blight infections as they may not always show signs of being infected by the blight causing pathogen. Infected parts need to be burned, buried or composted before planting in the spring.

When burying potato hulls, make sure that they are buried deep in the soil and they are covered with soil or plastic to block out any light that might help them photosynthesise and grow. When burning, pile them up in thin layers over combustible materials. Old specimens and infected plants can also be exposed to the extreme freezing temperatures of winter, if appropriate.

tomato plant infected with blight

In general, if the last option is chosen, it is recommended that the field in which exposure of the chopped cells to freezing temperatures should not be used for growing potatoes or tomatoes the following year - though growers may wish to include this technique as part of crop rotation and leaving fields fallow. Other nightshade plants should not be planted in a field that has been recently vacated by blighted potatoes, tomatoes or other crops.


Mechanical harvesters are notorious for promoting late blight infection. While moving through the soil in potatoes, they spread the spores that were left lying in the ground to tubers that were 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 line with their typical mode of transmission. If foliage infected with blight dies naturally, a period of at least fourteen days should pass before harvesting is allowed to commence. This is to allow room for the spores to die and not erroneously be spread by the harvesting process.

In general, the following steps should be taken to help prevent both early and late blights from transmitting and/or taking hold:

  • 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 Tomatoes

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

In terms of chemical treatment, spraying the infected tomato plants with Demildex® copper oxychloride or other copper-based fungicides is most effective. Spraying should continue to ensure the plant’s leaves are thoroughly wetted; this can take place alongside standard watering protocols. As with all methods of applying fertilizers, herbicides and pesticides, the optimal time to perform such duties is early in the day to negate the risk of the solvent immediately evaporating and causing inadvertent burning of the leaves.

For powdered copper fungicides, a thin layer of fungicide powder should be applied on both top and bottom of the leaves using a pressure duster. Reapplication of the fungicides every three to ten days is a standard practice,  until all blight symptoms vanish and the disease is cured. Additionally, spraying of the infected tomato plants with baking soda spray may be effective.

Blight Treatment for Potatoes

If few leaves are affected by blight, targeted removal and disposal of the infected leaves to limit the spread is often very effective. Otherwise, for a more severe infection, growers may 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.


tomato farm

You may also spray your crops using copper fungicides, one of the most effective of which is copper oxychloride. Copper oxychloride Demildex - a specifically developed fungicide - should be applied every 10 to 14 days following heavy rainfall or when the infection is rapidly increasing across the farm area. If possible, it is recommended that the fungicide is applied such that there are 12 hours of dry weather following application.

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.

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.

Notes On The Use Of Copper-Based Fungicides

Growers 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. In general, copper compounds are toxic to aquatic and mammalian life - therefore spraying should be performed with care to ensure no runoff to water courses and a reasonable time left between the last spraying and harvesting.


  • Blight can be fatal to an entire crop of tomatoes or potatoes and is broadly two diseases, early and late blight.
  • Early blight is caused by fungus Alternaria solani, whereas late blight is caused by oomycete Phytophthora infestans. Late blight is far more destructive to a crop.
  • Copper oxychloride Demildex is a highly effective fungicide against the pathogens that cause blight and can be used as part of a wider pruning and good crop management programme


Copper oxychloride Demildex, available exclusively from African Pegmatite is a highly effective fungicide, active against the fungi that cause blight in tomatoes and potatoes, and against many other fungi that are known problems for commercial growers.

copper oxychloride demildex used to treat black spots on roses