If your tomato plants are not looking healthy, they could be suffering from a disease called Late Blight (Phytophthora infestans), which also affects potatoes. This disease has been around for a while as it was responsible for the infamous Irish potato famine in the 1800’s.
Late blight is quite common this time of year. The obvious symptoms appear first on the lower, older leaves. Foliage develops large, brown blotches that soon turn droop, turn a light tan and become crispy. On the stems it appears as brown splotches. The tomato fruit develop dark brown circular spots that become mushy.
As the disease progresses, a white fungal growth appears on affected areas. This disease works quickly and soon entire plants succumb. The best thing to do is to pull out and discard infected plants and certainly don’t compost them.
Fog and morning dew promotes this disease, which we can do little about, however it is prudent not to get the foliage wet when watering. To prevent this disease, water in the morning, rotate crops, especially between tomatoes and potatoes crops. Wait at least a couple of years, preferably four, to ensure there is no cross contamination.
Uneven watering is the reason why cracks appear in tomato fruit. Dry soil followed by a thorough soaking makes the tomato fruit to grow too fast; too fast for its own skin. Thin skinned tomato varieties are especially prone to this condition. Try to keep the soil evenly moist. Container grown tomatoes benefit greatly from having a large drainage tray underneath them. A 3 inch layer of mulch on top of the soil, even ones in containers, helps to keep soil moisture more constant.
Blossom-End Rot appears on the bottom end of the tomatoes. It becomes flattened or sunken, brown and leathery. This is not a disease, but a lack of calcium in the soil. The two main reasons why the plant cannot absorb calcium are inconsistent watering and soil that’s too acidic.
Inconsistent watering, especially drought, prevents the distribution of calcium throughout the plant. Tomato plants grown in containers often suffer this malady as keeping the soil consistently moist is tricky during the heat of the summer.
Soil that is too acidic also prevents calcium absorption. Tomatoes prefer a pH of 6.2 to 6.8. Add dolomite lime to increase soil pH and as an added bonus, the lime also contains calcium. Work the lime into the soil, following the manufacturer’s instructions, then water well. Crushed eggshells mixed into the soil is also a great idea. Add them to your compost bin and apply the finished compost to your veggie beds and containers yearly.
To prevent this condition check soil pH in spring and apply dolomite or dolopril lime according to direction. Apply a mulch, keep soil evenly moist, avoid soil from drying out, avoid high nitrogen fertilizers as it ties up the calcium in the soil and avoid hoeing close to roots. If growing in pots, make sure the container is larger enough that it doesn’t dry out repeatedly and include a good sized drainage tray.
Wilting tomato plants may not be thirsty; they may be infected with the fungal disease, verticillium wilt. A good indication that the wilting is due to this disease and not a lack of water is moist soil (duh). Often only one side of the plant is affected and the plants recover at night.
Look for yellow blotches on the lower leaves that may curl inwards. As the disease progresses, the leaf veins turn brown followed by dead spots. This fungi prevents water from translocating throughout the plant and contains a toxin that kills and starves plant tissues as it spreads. Plants not only wilt and discolour, they become stunted and fruit, if any, are small, yellow and don’t develop. A good indication of this disease is inside the stem. If it has a dark centre when severed, it is verticillium wilt.
Tomatoes are not the only vegetable prone to this disease. All members of the tomato family are vulnerable: potatoes, peppers and eggplants (aubergines). Once a plant is infected toss it in the garbage as there is no control.
Since this fungi enters the plant through the roots from contaminated soil, remove plants as soon as they become infected. Remove all the plant parts from the area so they don’t re-contaminate the soil. Don’t put them in the compost. Rotate your crops. Don’t plant tomatoes, potatoes, eggplants and/or peppers in the same bed for at least 4 years! This is why I had to make another veggie bed, just so I could rotate my crops more efficiently.
Plant tomatoes in well-drained soil and don’t overwater as this encourages this disease. Just one day of saturated soil will incur infection especially if combined with cool soil: 13°C (55° F).
Plant resistant varieties. When purchasing tomato seeds and tomato plants look for the initial V at the end of the tomato’s name: Tomato - New Yorker (V). This indicates that the variety is resistant to verticillium wilt. That doesn’t mean it will not become infected if it is grown in contaminated soil. And, if all else fails, solarize the soil.
Decontaminating soil with Solarization
Soil contaminated with fungi, diseases, insects and weeds cause lots of heartaches for gardeners everywhere. Using the sun to 'bake' the soil is a non chemical, organic approach to kill soil nasties. Just till or dig the bed, rake level, water, level again ensuring a smooth surface. Place a sheet of clear plastic overtop and secure with pegs or soil from another non-contaminated bed. This is best done in the summer to heat the soil adequately. Keep them soil covered for at least 6 weeks.
Remove the plastic and add some goodies to replenish the soil since solarization kills practically everything. Add a few inches of compost, SeaSoil and/or composted manure to replenish what was lost. Mix it in to the top 6 inches and water. Wait a few days to a week to plant.
Here are some of my previous blog postings. They cover a wide range of topics from bugs to my botanical excursions and conventions. Click on whichever interests you on the titles below for easy navigation.