Beat Blight and Other Fungal Problems in an Organic Garden

Remember that prevention is always better than a cure.

tomatoes affected by blight


Blight is one of the most serious common fungal problems that you might encounter in an organic garden. So, how should you deal with this and other fungal problems? As a garden designer and consultant, I am often asked how to deal with this issue. The answer, as with so many things, is that prevention is better than a cure.

What Is Blight?

Blight is the name given to a range of fungal infections that can attack different plants. The type of blight most commonly referred to, however, is a blight which attacks potatoes, tomatoes, and other members of the same plant family, also referred to as "late blight."

Throughout history, many catastrophic crop losses have occurred due to this fungal infection, caused by a microorganism called Phytophthora infestans. It was implicated in famines in Europe in the 1840s, including the notorious potato famines in Ireland and Scotland.

Home growers in many regions encounter this issue, and while we don't usually depend today on a small number of crops, and so won't usually have such huge consequences as in the past, it can still lead to a dramatic drop in yields.

When blight occurs, fungal spores spread through plant tissue, creating brown areas on foliage which spread into a damp rot. It can spread from leaves to stems, blooms, and fruits or tubers if left unchecked. The spores linger in the soil for years, creating a problem that is difficult to eradicate from a growing area once it takes hold.

tomato leaf affected by blight

Yevhenii Orlov / Getty Images

Preventing Blight

While it can be very difficult, if not impossible, to eliminate the chances of blight taking hold entirely, you significantly reduce the chances of a severe infestation of this or other serious fungal issues by doing the following:

  • Placing plants in optimal locations with ideal growing conditions. The healthier plants are, the less likely they are to succumb to disease.
  • Growing blight-resistant varieties which, while not typically entirely immune, are less likely to have a serious problem.
  • Harvesting early, before late blight is likely to become an issue—for example, growing early potatoes to harvest before July.
  • Getting the spacing right with your potatoes, tomatoes, and other crops so there is good air flow between the plants.
  • Watering the soil at the plant's base, rather than from above; trying to avoid wetting the leaves as much as possible; and watering early in the day so the plant dries out more before nightfall.
  • Pruning lower leaves and using mulch to minimize spores splashing up onto plants.
  • Making sure that you don't grow potatoes, tomatoes, or other plants in the same family in the same location year after year. Practice crop rotation to reduce the chances of fungal buildup in the soil.

What to Do If Blight Occurs

If you do spot blight on your plants, it is important to act as quickly as possible to reduce its spread. The more quickly you remove affected material, the greater chance there is to contain the outbreak.

Take any affected material and dispose of it. Keep it well away from your garden and composting areas. Practice good hygiene, washing your hands and gardening tools so you don't spread the disease to other plants in your garden. 

If the problem has not spread too much, you can consider spreading antifungal treatment on other currently unaffected but susceptible plants nearby. For example, you might use a spray of bicarbonate of soda in water (10 grams to 1 liter of water), applied carefully above and below all leaves early in the morning. There are also fungicides containing a bacteria called Streptomyces lycidus that you might use.

This and other such sprays won't eliminate all risk and are not entirely effective. But they could reduce the chances of the problem spreading somewhat.

If the problem has spread too far, unfortunately, such measures won't do much to help. That is why it is so important to remain vigilant and to act quickly. 

With potatoes in particular, there is one more drastic step that you might be able to take to save at least some of your crop. If more than a quarter of the leaves on potato plants show blight, cut off all the stalks at ground level, removing all the material and disposing of it right away. After a couple of weeks, you can then dig up your potatoes; these may still be unaffected and you will be able to eat or store them as usual.

If you have severe blight or other serious fungal problems on tomatoes or other crops, you may lose the entire yield this year. But by taking the steps for prevention mentioned above, you should reduce the chances of the same thing happening next year.