Let’s get one thing out of the way right off the bat.
If your tomato plant starts wilting, the leaves go brown and start rotting all within 2-3 days, you may have Tomato Late Blight.
The fruit will be rotted as well so there’s no question of eating it.
If the leaves develop brown spots or lesions but the entire plant doesn’t wilt and die very, very quickly, and you can still continue to harvest the tomatoes after a few days while (mostly) the lower leaves go bad – you don’t have Late Blight.
Understand the Late Blight parasite can only survive in living tissue. This means your first step is to remove all tomato, pepper, eggplant, and potato plant debris from the property.
If you allowed fruit to fall on the ground, it must be removed. If you grew potatoes, all of them must be removed from the ground.
Do not compost the remains *unless* you have a very hot compost pile (it’s safer to remove them from the property)
Diagnosis of Late Blight versus other Blights and Problems This is a Cornell website with excellent pictures about the various tomato diseases that gardeners may think is Late Blight.
You have to practice crop rotation. None of the Solanacea family (plants above) should be grown in the same spot – they all need to move every year.
They require full sun and excellent air movement around the plants. I’ll be coming back to air movement so mark this requirement as special.
The plants require a well-drained soil. Constantly damp soils encourage the Phytophthora infestans disease. If you have damp soils, consider building raised beds to get your plants above the soaked soil.
Seed Treatment for Tomatoes
None – late blight in tomatoes is NOT a seed problem so you don’t get it from bad seed.
Most of the resistant varieties are commercial crops at this time but stay tuned for developments and news releases on these.
Matt’s Wild Cherry may have some resistance and Stupice and Juliet may as well.
If you live in an area where Late Blight has been an issue, plant nothing but the earliest tomatoes you can find. One hope is they will have grown and fruited before the problem becomes acute later in the season. You may very well get the majority of your crop.
Consider very early plantings and very late plantings so the problem may hit one but not the other planting depending on local weather conditions.
Separate all the susceptible plant families so the disease doesn’t jump and spread to your entire garden.
When Do You See the Problem.
This disease becomes a problem mostly in times of when the leaves stay constantly damp. Damp leaves are a perfect breeding ground for this problem.
- Grow nothing but staked plants. This allows the leaves to dry out because of the spacing and wind flow. Prune off bottom foliage below the fruit set that is currently ripening – leaving only the foliage above the ripening fruit set.
- Grow in plastic tunnels. Construct small greenhouses over your plants to protect them from excessive rain.
- Do not water with a sprinkler system or hose. Drip irrigation only.
- Orient your rows to the prevailing wind direction (normally from the southwest) so the wind blows between the rows to dry out the leaves.
- Plant non-susceptible plants between the susceptible ones (corn between rows of tomatoes or between the peppers and tomatoes) This may stop the problem from easily jumping between rows of susceptible plants.
- Do not use a lot of nitrogen in your feeding regime. Excess nitrogen produces fewer tomatoes to start with and also produces a full, lush plant growth. Lush plant growth is very susceptible to a lot of other problems as well as the Late Blight.
- Weeds stop air flow in the garden. Remove them. That takes care of that.
- Pull any volunteer tomatoes.
- Do not harvest tomatoes or work in the tomato patch when the leaves are damp. The spores will easily move from plant to plant on your damp clothing or tools.
- After harvesting is done – and your plants are healthy – remove them to the compost pile. If diseased, remove them from the property or ensure a hot compost pile to kill off pathogens.
Can I spray?
- Copper has been shown to slow down Late Blight. And it is considered “organic” under restricted conditions. But the deal is simple – you have to apply the copper spray to the leaves “before” they get infected. Once they are infected, no amount of spray is going to do a darn thing- organic or otherwise.
- The entire plant – tops and bottoms of leaves must be thoroughly coated. Note the Phytophthora loves the underside of leaves and mostly starts there.
- Please understand that copper sprays will not likely kill or stop the pathogen totally. In research trials, it tended to “slow it down” by two to four weeks.
- There is a tremendous amount of controversy about whether compost tea has any effect on this problem. Bottom line – if your compost tea contains a microorganism predator that eats this Phytophthora then it might. But don’t count on it. There’s a lot of misinformation running around the Net about this – take it all with a grain of salt (or two grains).
- Copper is a “restricted” product in organic gardening. It is only allowed if the correct amounts are used and accumulation in the soil does not happen. In other words, use it only if you really had Late Blight and not just “in case” you might. Your best defense is still cultural.
- Note that copper poisoning is quite possible in humans and all safety precautions have to be made if you decide to spray with a copper based alternative such as Bordeaux mix.
I Have A Late Blight Plant – Now What?
If you see a plant with Late Blight and it’s collapsing – immediately pull it from the garden and remove it from your property. It’s probably too late anyway but you might delay other plants becoming infected until you can get some fruit from them. If it appears to be spreading – harvest all your green tomatoes from healthy plants and learn to make green tomato chutney.
Commercially – trials indicate that a single diseased plant can infect other tomatoes within 100 feet.
Here are other organic vegetable gardening tips