Ask Ruth: Preventing Tomato Blight and Resistant Varieties

Hi Ruth,

After this wet summer, I’m wondering if any varieties of tomatoes showed any resistance to the blight (early or late). I grew Opalkas, Amish paste, and Juliets for canning; Cherokee pink, Brandywine Black, Eva Purple Ball, and Early Girl for eating; and Sungolds and Black Cherokees for cherry tomatoes. Here at the end of August I picked a few cherry tomatoes, but all others are gone. Juliets lasted the longest. So what to do next year???

I never grow tomatoes in the same place or where potatoes were. I’ve even tried growing tomatoes outside of the garden in new ground. I use fresh compost every year and I don’t till. I put landscape cloth beside my plants to reduce weeding and prevent soil splash. I’ve tried Serenade alternating with copper to help prevent blight. What else can I do?

Thanks!
Pat
Madison County

Hello Pat,
You are an excellent and very experienced gardener, and it sounds like you are already extremely pro-active regarding tomato blight prevention. Like most Western North Carolina gardeners, you have been facing the devastation of tomato blight for years on end, and it is a super-frustrating disease. I had never experienced tomato bight when I moved here 22 years ago. During my first gardening year in the mountains, my tomatoes were almost six feet tall and beautiful at the beginning of August. When I returned from a vacation two weeks later, my tomatoes looked like they had been sprayed with an herbicide (they were mostly a dead brown color). It was my first shocking encounter with tomato blight. Ouch!

Tomato blight is merciless. Don’t take it personally. I talked to a few organic farmers at the Montford Farmers Market and their conclusions match my own conclusions. With tomato blight in WNC, it is not a question of if you will get blight, but more a question of when you will get it. We talked various strategies for staving it off (some below). Not very encouraging is it?

OK. SO WHAT IS TOMATO BLIGHT?

Early blight of tomato, Alternaria solani, is a fungal pathogen that appears as concentric spots on the foliage and can eventually rot out the stem. Prevention is the key.

Late blight of tomato, Phytophthora infestans, is caused by a fungus-like pathogen, the same one that caused the Irish potato famine in the 1840’s. This serious disease can be spread by infected transplants, and by volunteer tomatoes or potatoes and certain related weeds. The airborne spores can travel 10+ miles, and spread via wind and rain. This summer’s weather was especially favorable with lots of rain and cool, overcast days. The first signs of late blight are irregularly shaped, water-soaked lesions that usually have a lighter colored ring around them. In bad years, a crop can be destroyed in less than two to three weeks. Prevention is the key.

SOME CONSIDERATIONS FOR GROWING TOMATOES ORGANICALLY IN WNC

Choosing Tomato Plants: Buy healthy transplants and check transplants for signs of blight before you purchase them.

Choose Fast Growing Tomatoes: Choose plants that mature quickly, like Early Girl* (harvest tomatoes before blight arrives). Or choose plants that will grow quickly – mostly hybrids, any fast-growing tomato, or grafted tomatoes – so that they can outrun the blight (the top tier will be producing while the bottom tier is blighty). *This strategy works some years, but this year both Pat and I had bad luck with Early Girl.

Choose Blight-resistant Tomatoes: Blight-resistant does not mean blight-proof, it means that blight is slowed down so you will get a longer harvest. Resistant tomatoes are bred to resist particular strains of blight. If that blight strain mutates or a new strain shows up, we’re back to the drawing board. Some suggestions by locals were:

• Mountain Magic F1 Hybrid – Indeterminate, small red tomato (larger than a cherry, smaller than a plum). Vanessa Campbell says it is one of their favorites; it tastes great and is good for everything except slicing for sandwiches. A Randy Gardener/NC State tomato.
• Plum Regal F1 Hybrid – Determinate, red plum tomato. Another Randy Gardener/NC State tomato.
• Matt’s Wild Cherry – Heirloom, Indeterminate. Red fruit. Was listed in multiple places. EB resistant, moderate LB resistance.
• Iron Lady F1 Hybrid – Vanessa had good luck with Iron Lady. My Iron Lady got blight (I did not spray) but it produced a lot of tomatoes. I thought they were good, but one of my friends thought they were mealy.
• NCSU 144 – Tom Elmore had good luck with this pink tomato from NC State. I planted it last year and it did seem blight resistant, but was not very productive for me.
• Defiant F1 Hybrid– Determinate, Mid-sized red fruit. Harry Hamil’s friend had good luck with this and Johnny’s catalog devoted an entire page to this tomato. I would love to try it.
• Big Beef F1 Hybrid – Large red fruit, Indeterminate, Jenn Cloke has good luck with this tomato in both wet and dry years (but she adds that she uses “nuclear amounts of copper”).
• Legend F1 Hybrid – Determinate, large red round fruit, early, OSU
• Mountain Merit F1 Hybrid– determinate, a large-fruited red variety with resistance to some strains of late blight. A Randy Gardener/ NC State tomato.

Scroll to page 4 of this link for more blight-resistant tomato variety ideas. Unfortunately Pat, some of these listed varieties didn’t work well for you this year, like Juliet.

Fungicides for Early & Late Blight: Approaches for early and late light are very similar. Basically, especially for organic gardeners, both early and late blight must be prevented, as it cannot be controlled once you already have blight. This means spraying weekly or more often with an organic fungicide, and it is a good idea to start your spray regime at planting time. You need to spray often enough to protect new foliage, and Dr. Jeanine Davis recommends re-spraying after a rain. Fungicides should thoroughly coat the plant on all sides of the leaves and it is best sprayed early in the morning when the plant is wet from dew anyway. I like using a sprayer with a wand so you can easily reach the undersides of leaves and the insides of the plant. Most copper formulations are considered dangerous so – with any and all pesticides – always read and follow all safety, re-entry, safe harvest, and other instructions.

Full Sun Farm alternates between the fungicides Oxidate and Actinovate, but they did not spray this year because it seemed hopeless with all the rain. Next year they may add copper to their spray rotation. The farm manager for Johnny’s Selected Seeds names Oxidate as his favorite tool to use for late blight.

Jeanine Davis recommends spraying every 5 days alternating copper and Serenade (Bacillus subtilis), Sonata (Bacillus pumilus), or Sporatec (mostly herbal oils). She has experimented with using Serenade + copper for one spray and then Sporatec + Neem in the next spray but now is unsure about combining copper with Serenade in the same spray because there is some controversial about whether the copper inactivates the Serenade or not. Read more here.

Another popular regime is alternating Senenade (Bacillus subtillus) with copper. Regime:

• Week One – spray Serenade;
• Week Two – spray Serenade;
• Week Three – spray copper;
• Repeat sequence until frost.

Firefly Farm grows a number of heirloom tomatoes under cover. They use Serenade and they also spray with hydrolyzed fish and liquid seaweed. Scott does not use copper.

Copper is often considered the most effective fungicide for blight, however copper is a heavy metal that can accumulate in the soil. There are some concerns about copper’s effects on human health and wildlife including earthworms and bees; it is highly toxic to fish, and has even been banned in the Netherlands and Denmark. According to eXtension, “As required by the certification process, farmers must use all available alternative practices to manage late blight, and describe these in the Organic System Plan, before deciding to apply a copper product.” Certified growers should consult with their certifier before applying copper.

CULTURAL PRACTICES to Help Tomatoes Resist Blight

• Mulch – to keep the pathogen from bouncing up from the soil and on to the plant.
• Remove Volunteers – Immediately remove volunteer tomatoes and volunteer potatoes for best blight prevention.
• Air circulation – Plant tomatoes in an area that has good air circulations with consideration given to prevailing winds. Keep area weed-free to encourage good air circulation. Sucker tomato plants so they are less bushy and will allow more air to circulate.
• Water – Water in early morning at the base of the plants (not with a sprinkler), or use drip tape. Keep the foliage dry. Tomatoes require 1” of water per week. Don’t let your plants become drought-stressed.
• Fertilize – Apply adequate amounts of compost or fertilizer to keep your plants growing nicely, but don’t over-fertilize as that can cause other problems. You want enough new growth that the tomato plant stays ahead of the blight.
• Prevent Spread of Disease – Don’t work your plants when they are wet as this will quickly spread the disease. Harvest at the end of the day when foliage is dry. Keep hands and tools clean. Work blighty plants last, so you don’t contaminate good plants. Scout for blight every few days.
• Remove Blighty Material – Remove blighty leaves from the plant, being careful not to touch other parts of the plant. Don’t compost blighty tomatoes, leaves, or vines. These should be bagged and removed from the property or deeply buried. Clean up well in fall.
• Rotate Your Crops – Don’t plant nightshades (tomatoes, potatoes, peppers, eggplants, etc.) in the same area next year.
• Save Your Best Seed – If you plant heirloom tomatoes, save the seeds from the tomatoes that show the most blight resistance. For the last ten years, Scott Paquin has been selecting seed from his Cherokee Purple tomato plants based on their most blight resistance traits. Don’t bother saving seed from hybrid tomato plants since they will not be true to type.
• Keep Your Plants Healthy – so they are less susceptible to disease.
• Store Blighted Fruit Separately From Good Fruit – The pathogen can move from the blighty fruit into the good fruit. Blight spots on tomatoes are darkish-colored and have an unpleasant odor. You can cut the bad spots out and still eat the tomato – up to a point.

TUNNELS AND OTHER IDEAS

Many local farmers raise their tomatoes in a high tunnel that has been designed for summer ventilation with roll-up sides and good ventilation capability on each end. This keeps the plants dry, and dry plants are way less susceptible to blight. Drip tape is installed for irrigation right at the base of the plant. In his tunnel, Scott Paquin has good luck with Marglobe and Cherokee Purple, both heirlooms.

Jeff Ashton described a contraption that Dr. John Wilson uses to keep his tomato plants dry. He installs a tall 2 x 2” stake (about 8’ tall) in the middle right next to the tomato. He adds 4 shorter poles around the outside of the tomato and covers the whole business with greenhouse plastic – essentially creating a tent to keep the tomato dry. The plastic should not touch the plant. He probably pads the top of the poles a little to keep the plastic from tearing. This is a bit funkier than a high tunnel, but it also requires a lot less investment. You will need to water at the base of the tomato plant.

Tom Elmore suggested another inexpensive idea for keeping your plants dry. Plant your tomato in full sun on the south side of the house under the cover of the roof eave. Apply fungicidal sprays (Tom suggests copper), and water the tomato plant at the ground level. I think tomato blight will always be a challenge in the southern Appalachians, but hopefully this was of some help to you Pat – and I hope you have better luck with your tomatoes next year.

Special thanks to Vanessa Campbell/Full Sun Farm, Christina Carter /Ten Mile Farm, Jenn Cloke, Tom Elmore/Thatchmore Farm, Harry Hamil, Scott Paquin/Firefly Farm, and Pete Shriner/ Blue Ribbon Farm for their thoughts on blight/blight-resistant tomatoes.

Thanks for writing and happy fall gardening,
Ruth

Ask Ruth © 2013 Ruth Gonzalez & Organic Growers School