How To Prevent Tomato Blight At Home Effectively

There are many different strains of this pathogen. Some can infect both tomatoes and potatoes, but others are specialized for one host or another.

Conditions That Favor Late Blight

Once again, there is a reason this type of pathogen is called a water mold. The organism requires humidity greater than 90 percent to cause infection and moderate temperatures in the range of 60 to 68 °F.

If the spores are present under these conditions, they can infect a plant in about 10 hours. The infection spreads rapidly and plants can quickly succumb, in a matter of days.

Spores can spread long distances on the wind, particularly in cloudy conditions.

Although infection may occur more slowly, if nights are moderately warm and conditions are extremely humid but hot during the day, the disease can still develop in temperatures as high as 95°F.

Warm, dry conditions help stop the spread of disease.

Identify Late Blight on Tomato Plants

This disease tends to occur later in the season than the similarly named early blight, usually after plants have flowered and when weather conditions are favorable.

The first indication that your tomato plants are infected will be evidenced by small water-soaked lesions on the leaves.

These will quickly grow into large, irregular purple-brown spots that may appear oily, with a gray-green border.

Look at the underside of the leaves with a magnifying glass. Under ideal conditions, the pathogen creates rings of grayish-white fungal threads, and spore-producing structures develop around the spots.

Entire leaves will turn brown and die, and the infection will spread rapidly to petioles and young stems. At this stage, the stems may continue to shed spores for longer periods than would be the case if only the leaves were infected.

If conditions are no longer favorable for further spread of the disease, lesions will have a brown dead center surrounded by tissue that has collapsed and may appear grayish-green, yellowish, or water-soaked.

The fruit can also be infected and usually shows symptoms first on the shoulders. This happens because the spores fall onto the fruit from above.

The fruit will turn brown with greasy-looking spots, but will remain firm unless infected by other organisms, such as soft rot bacteria, which can penetrate damaged plant tissue. These brown patches can grow in size as the infection progresses and become leathery.

Both unripe green fruit and fruit that has ripened on the vine can be infected. Although the unaffected parts of the large fruit may still be technically edible, eating them is not recommended.

Any infected portion or severely damaged fruit should be discarded. Other pathogens may be present, giving the fruit a bitter taste and causing it to continue to rot off the vine.

Monitor for Disease and Scout Your Garden

Experts strongly recommend that you scan your garden at least twice a week for symptoms of this disease. You should check more frequently during periods of wet weather.

Late blight can infect some parts of gardens while others are spared. Begin your search efforts in wet areas, including:

• Low spots in your garden
• Densely planted crops
• Planting areas next to forests or hedges

If you identify late blight, notify your county extension agent. Cases of this disease are monitored in local regions and alerts may be available to growers when favorable conditions are forecast.

USABlight, an online portal created at North Carolina State University, allows registered users to check infection reports in their local area, and can also report local disease occurrences or find information to submit samples for testing.

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How to Manage a Late Blight Infection

In an ideal world, your best bet would be to grow late blight resistant tomatoes.

However, the oomycete continues to evolve and outgrow these traits in various hybrid varieties bred for resistance.

For a time, there were no commercially available tomato cultivars bred for late blight resistance, but many varieties are now undergoing breeding trials.

Unfortunately, this resistance can be broken if conditions are too favorable for infection.

Heirloom saved seeds should be safe to use, if they have dried completely. Note that saved seeds of hybrid varieties will not grow true to the parent plant.

Only buy plants from high-quality suppliers, preferably those that guarantee that their plants are disease-free. And be sure to inspect all plants for signs of disease before putting them in your garden.

In addition to planting resistant varieties, you may consider using fungicides to treat minor infections, depending on disease progression and local weather conditions.

I’ll cover how to determine if this is a viable option a bit later in the article. But remember that this disease spreads quickly under the right conditions and there is no cure.

If you find evidence of late blight that is already serious, remove and destroy diseased plants immediately. Unload them in their entirety, cover or cut them at the soil line and immediately bag them, so that the spores do not spread.

Ideally, you should destroy the plants on a dry, sunny day, so that the spores die quickly. But if the weather will remain wet for a while, it is better to destroy the plants immediately than to wait for a dry day.

As described above, potatoes are also very vulnerable to late blight. The organism can also attack other solanaceous plants, including hairy nightshade (Solanum physalifolium) and greenhouse-grown petunias.

Infected weeds can contribute to further spread.

Remove hairy nightshade weeds from your garden and surrounding areas, and keep a close eye on your petunias for signs of infection if they’re growing nearby.

Be sure to rotate nightshades with other crops from season to season when growing them in your garden, and check to see if they spread to other vulnerable plants if they’re growing in close proximity.

Above all, prevention is key.

How To Prevent Tomato Blight

There are several steps you can take to prevent late blight from taking hold in your garden. These include:

• Removing any volunteer tomato and potato plants, and any wild nightshades, from the garden.
• Check starts before you plant them, to make sure they are free of disease symptoms.
• Avoid overhead sprinkler irrigation, and only water at the base of plants.

Removing weeds and volunteer seedlings is particularly important, since the pathogen can overwinter on tomato seeds as well as potato tubers, and produce infected seedlings the next season.

Proper watering is a precautionary measure that can be taken to prevent all sorts of fungal diseases. Adequate airflow is also important, to reduce humidity and give your plants a chance to dry off after periods of rain.

Be sure to provide proper spacing between plants, as recommended for the cultivars you have selected.

Some gardeners like to remove the lower leaves from their tomato plants, effectively preventing any foliage from touching the surface of the soil.

This can also help to prevent fungal disease, further supporting proper airflow and allowing wet foliage to dry more readily.

Some Drugs Fungicides

Fungicides can be used to relieve early symptoms, but there is no cure for this disease. Check with your county extension agent to find out which fungicides are most effective against strains of late blight in your area.

If you choose this route, plan to use fungicides preemptively when rain is forecast, or if you are using overhead irrigation.

If a mild outbreak occurs, it is important to reapply fungicides at regular intervals for maximum effectiveness.

Frequent low-dose spraying has been found to be more effective than more frequent spraying with highly concentrated doses of fungicides. Be sure to cover the plants in their entirety, or the chemicals may not be as effective.

Bonide Fung-onil Concentrate

Chlorothalonil is a common fungicide that is sometimes used to control late blight. Please note that this chemical has been banned in some areas, and proper precautions should always be taken to protect yourself and the local environment when applying chemicals in the garden.

It is important to rotate the use of fungicides or apply them in combination as recommended by a local extension agent to avoid fungicide resistance that will render these products ineffective.

Why Water Mold/Fungal Sex Matters

Perhaps you don’t think very often about the sexual life of fungi and water molds. But there are actually some really practical reasons to be concerned about this phase of their life cycle.

Microbes that reproduce sexually are much more likely to produce new mutations than those that reproduce asexually. And this matters, because this is how quickly new races of fungi emerge that are more aggressive and resistant to certain fungicides.

Although asexual reproduction is more typical, sexual reproduction sometimes occurs in these organisms.

Strains of late blight were once known to only mate sexually in Mexico, not because it’s a nice vacation spot, but because this was the original source of the pathogen.

Many of the newer mutants can be traced back to this part of the world.

This has had serious implications for late blight control in the US among farmers and home gardeners, as resistance to a number of once highly effective fungicides developed in the 1980s, rendering them useless .

Now, strains capable of mating also exist in the US, which may hasten the development of new strains of this deadly pathogen.