Vegetable garden plants are prone to suffering from what is commonly referred to as blight. Blight is a broad description consisting of symptoms caused by many different plant diseases. Basically, yellow, spotted, brown, dead, or necrotic leaf, stem, or fruit tissue can be described as a blight. Where the blight comes from or what the main cause of it is will dictate how to best manage that blight. But remember, not all plant diseases are blights.
There are two main types of blight that affect tomato plants.
- Early blight (EB = resistance code)
- Late blight (LB = resistance code)
Although these two blights are similarly named, they are caused by two separate fungal pathogens, which have different symptoms and behaviors. Not surprisingly then, combatting these two tomato blights is different, because of how the pathogens behave. Below are symptom differences to help determine which disease you're dealing with.
Early Blight - EB
- Early blight (Alternaria solani) will typically show up on older leaves near the base of the plant.
- It is most commonly spread from infected soil debris or insects feeding from plant to plant.
- Infections start modestly and can easily escape detection.
- Leaf spots of ¼ to ½ inch form first with tan dead tissue in the center often showing concentric rings of increasing damage and a yellow ring or halo around the outer edges of the spots.
- These spots keep growing, eventually congealing into a larger mass of dead leaf tissue.
- Stem lesions will be brown, sunken, and girdled.
- Early blight affects plants more slowly than Late blight does.
- Slow enough that if plants are kept adequately watered and fertilized, they can grow through infection to continue to produce flowers and fruit.
- Determinate varieties are less susceptible than indeterminate varieties.
NOTE: The rings make the spots look somewhat like a target.
Late Blight - LB
- Late blight infects rapidly and affects plants seemingly overnight.
- Similar water-soaked spots show up first on lower leaves but quickly jump to infect upper, younger leaves too.
- During Late blight infections the tan to brown spots will not show a yellow halo between the necrotic tissue and healthy green leaf tissue, nor will the spots show characteristic growth rings.
- Sometimes during LB infection white fuzzy mold can be visible on the stems and underside of the leaves.
- While some resistant varieties exist, because of the potential severity of Late blight, constant monitoring, sanitation, and vigilant fungicide applications become highly beneficial for prevention.
Blight Control & Management
Late blight (LB) is the worst or most dreaded of these two blights. For some historical context, the pathogen responsible for Late blight is the same pathogen that led to the great potato famine in Ireland during the mid-1800s.
- Once a positive ID of Late Blight has been made, the recommended control is to simply remove infected plants.
- Remove and destroy plants during the heat of the day, during dry conditions.
- Do this as carefully as possible making sure to isolate infected plants and debris from any healthy tomato plants to limit the spread.
Early blight (EB) infected plants should be kept as healthy as possible.
- Water consistently, apply fresh and ample mulch around plants, and fertilize properly.
- Bottom foliage should be removed as soon as spotting is detected.
- Always adhere to proper sanitation practices when doing this type of chore, meaning don't work with plants if the foliage is wet from rain or dew, and put leaf debris directly into the trash. Wash your hands/gloves and keep pruners clean, especially before working with non-infected plants.
Prevention of Infection
As with any fungal disease, prevention is the key to control. Follow all the guidelines below.
- Prevention starts with planting resistant tomato varieties. Pay attention to the provided disease codes: EB or LB, which are self-explanatory. 'Mountain Merit', 'Mountain Magic', 'Plum Regal', 'Defiant', and 'Jasper' hybrids are all examples of varieties that show resistance to both blights.
- Never compost plants or fruits suspected of being infected by these two blights or any other diseases. The infecting pathogens will not die during composting and can easily re-infect new plants the next season when that compost is used. This is a common gardener error that is preventable.
- Prune off lower tomato leaves to keep them at least 1 ft. off the ground. This helps prevent infection from splashing water and helps reduce ground humidity while improving air circulation.
- Keep 2 to 3 inches of clean mulch around the base of your plants to aid in moisture retention and to also help reduce unwanted water splashing.
Utilizing fungicides for prevention can also help in reducing potential infections of these two blights, however, specific rules must be followed to not waste your time, efforts, and money.
NOTE: Because of the nature of Phytophthora infestans that causes Late blight, most fungicides will not prevent it. The active ingredient chlorothalonil (Fung-onil by Bonide®) or Bio-fungicide (Zonix™) that controls Phytophthora spores may aid in prevention later in the season if they are used according to label instructions and other prevention methods are followed.
NOTE: Late blight infections are not severe every year. During some growing seasons Late blight is of little to no concern because of its dependence on specific weather conditions to spread.
- Fungicides for preventing Early blight include chlorothalonil, copper types, and bio-fungicides like Revitalize® by Bonide with Bacillus strain-D747. Application frequencies and rates must follow the different product label instructions, as these will all vary some product to product.
TIP: Neem oil has low to no effectiveness against fungal diseases like Early or Late blight.
The best disease prevention is achieved when at least two different fungicides are used in rotation. When the same synthetic fungicide product is sprayed regularly, pathogens can develop resistance and immunity to that product, rendering it ineffective long-term.
If your garden plants have suffered from disease a few years in a row, and you have been applying the same fungicide to combat those diseases with little to no apparent effect, then you should apply a different type of fungicide and rotate the type of fungicide each time you spray.
Pathogens cannot develop resistance to mineral-based fungicides like copper or sulfur or bio-fungicides, but rotation applications can still be helpful to protect plants.