Thanks for your question and for thinking ahead for next growing season.  I have found two sources that address your concerns.  From your letter, I feel you are aware of the issues.


https://permies.com/t/164822/rid-soil-tomato-blight
How to rid soil of tomato blight?
My urban garden has been consistently productive for most of the last 20 years. Gardening in the city means I have just one small area that has sufficient sun light. So I have ignored the rule of rotating my tomato crop and the tomatoes get the sweet spot in the garden every year. Well blight finally caught up to me. Last year I noticed a little spotting and leaf curl and when I came back from a 2 week vacation my tomatoes were all but dead. I removed all the tomatoes from the garden and added 6 to 8 inches of wood chips in the fall. This year I planted as usual just moving the chips enough to plant and then replacing the chips without touching the plants.
You guest it, my blight problem is still a problem. I have been spraying a mix of baking soda water and a drop of dish soap on both the top and bottom of the leaves and the wood chips as well. All for nothing, again my tomatoes are all but dead.

Is there any way to rid the soil of the fungus so maybe next year I can grow some tomatoes?Anne Millermaster steward

Posts: 19819Location: USDA Zone 8a4974 I like...
From what I understand this will remain in your soil for 3 or 4 years. Dispose of diseased plants in plastic bags.

Buy tomato blight-resistant tomatoes and plant them in a different spot in your garden.

Maybe it would be wise to trim some trees to give more sunlight to your garden.

Invasive plants are Earth's way of insisting we notice her medicines. Stephen Herrod Buhner
Everyone learns what works by learning what doesn't work. Stephen Herrod Buhner

Chris KottpollinatorPosts: 3847Location: Marmora, Ontario599 4I like...
My understanding of the situation is similar to Anne's. Ridding soil of a specific fungus isn't easy, and it's impossible if you keep feeding it by continuously planting their food in the same spot, season after season.

Now you could opt for radical interventions. Some would suggest removing the problem soil, bringing in new, or shifting existing soil around. My feeling about this is that you'd probably end up with tomato blight everywhere.

I suppose one could consider solarizing the spot for a season. Black tarps, EPDM, or plastic garbage bags have been used in the past to great effect to cook soil underneath. I haven't had the chance to experiment, but the next time I have the chance, I want to try using something porous and breathable, like blackout curtains, to see if it would also work.

In an urban environment, this might actually be a valid option. You'd want to stick to tarps, but you could probably plant your tomatoes out in five gallon buckets or whatever you have and sit them on the corners and edges of the tarps, at generous regular spacing. I wouldn't overcrowd the buckets because you'd shade out the whole tarp, which would mean not achieving solarisation temperatures.

Incidentally, the recommended bake time for soils and composts exposed to or infected by Phytopthora infestans is 120 °F (49 C) for 30 minutes. If you used a thermal probe or compost thermometre placed in the soil to a depth of 12", you could check the temperature on hot days. Theoretically, should it exceed that temperature for that period of time, the soil should be sufficiently solarised.

Do I believe that would actually work? Honestly, I feel that some foamcore insulation would be required along the perimeter to trap that kind of heat. To do it properly, I'd add glass, or better yet, a cold frame, atop the tarps, and a tightly-pegged sheet of plastic greenhouse sheeting tented over that.

One option I would consider strongly is that of fungal slurry treatments. I think I would take a known aggressive culinary variety, like Oyster mushrooms, or maybe winecaps, make water and mushroom slurries of them and inoculate piles of woodchips. When I saw some myceliation of the wood chip pile, I would spread the pile out over the bare infected soil and add more slurry. I would probably hold off on the application of actively aerated compost extract until I saw some establishment of the slurry fungi in the mulch and into the soil. The compost extract would then go to feed the fungi in their fight against Phytopthora infestans.

I would also grow it out. Decide what else you could grow there that you also eat and delight in, and put your tomatoes elsewhere. Maybe some earlier-ripening or smaller varieties in containers would do until you could use your tomato patch again. For my much better half and I, that would probably be something like butternut squash, but it might be a stand of Russian Mammoth sunflowers, or I might grow an asparagus guild heavy on tomato companions, including diverse alliums and marigolds. By the time the tomatoes return, it would be a polyculture with an active and aggressive fungal component in the soil layer ready to compete with the blight for resources, just waiting to welcome tomatoes back into the fold.

I don't see any viable method other than solarising it, checking temperatures with a thermometre or temperature probe, that would kill the pathogen, along with all soil life, and shorten the time you have to wait to grow tomatoes in that spot again. I would even consider hammering sheets of corrugated metal sideways into the soil, such that they act as heat exchangers. Hot metal fins extending down into infected soil, heated and kept from cooling to the air by a cold frame, or even a black tarp; that might effectively and evenly heat sterilise the soil.

Honestly? I would plant something else, even just cycling good green manures, and do the slurry thing as described. Planting something that generates a lot of biomass and root zone exudates that are food for the blight's microbial competition helps the soil to outcompete it. In the meantime, grow tomatoes in another spot, perhaps in containers somewhere, or planters, which might isolate plants and soil from the existant fungi you're having issues with.

But good luck. All tomato diseases are plagues of mythic proportions. I once crowded 39 tomato plants of different varieties into the sunniest, breeziest spot in all my garden, gave them all they needed, and got so pissed off when the neighbour's blight spread to my garden, I dug a 7'x17'x3' deep trench and built a hugelbeet out of it. That was one hell of an afternoon.

https://yardandgarden.extension.iastate.edu/faq/how-can-i-prevent-blight-my-tomatoes

How can I prevent blight on my tomatoes?

Question:

How can I prevent blight on my tomatoes?

Answer:

Foliar diseases of tomatoes are a common problem in the home garden. Fungal diseases, such as early blight (Alternaria tomatophila and Alternaria solani), late blight (Phytophthora infestans), and Septoria leaf spot, overwinter on plant debris in the soil. Fungal spores are splashed onto plant foliage by raindrops or splashing water. A wet leaf surface is required for the spores to invade the plant tissue. Wet spring and early summer weather favors development of foliar diseases on tomatoes.

Home gardeners can reduce blight problems on their tomatoes with good cultural practices.

Rotating Crop Location

Plant tomatoes in a different location in the garden each year. Rotate crops so that tomatoes and other solanaceous crops (potatoes, peppers, and eggplants) are not grown in the same area for at least 3 or 4 years. A 3 or 4-year rotation may not be feasible for gardeners with small vegetable gardens. However, small-plot gardeners should rotate as much as possible. No home garden treatment effectively destroys the disease pathogens in the soil.

Air Movement and Wire Support

When planting tomatoes, space plants approximately 3 feet apart. Wide plant spacing increases air movement and promotes rapid drying of plant foliage.

Grow tomato plants in wire cages. The foliage of tomatoes growing in wire cages dries more quickly than those sprawling on the ground. Gardeners can buy tomato cages at garden centers or make their own using concrete reinforcing wire or hog wire. A wire cage 2 feet in diameter and 4 to 5 feet tall should be adequate for most tomato varieties.

Applying Mulch

In early June, apply a 2 to 3-inch layer of mulch around each tomato plant. Shredded leaves, dry grass clippings, and straw are excellent mulches. The mulch reduces the splashing of fungal spores on plant foliage. Mulching tomato plants in early June allows the soil to warm up in spring.

Avoid Overhead Watering

Avoid wetting tomato foliage when watering. Apply water directly to the ground around plants with a soaker hose, slow-running hose, or watering can. If a sprinkler must be used, water in the morning so the foliage dries quickly.

Good Cultivar Selection

Some success can be found in planting varieties that are more resistant to blight diseases. There are not many varieities that show good resistance, but selections like 'Defiant F1', 'Seiger', 'Summer Sweetheart', and 'Chiapas' have shown some resistance. An extensive list of disease-resistant tomato varieties can be found in this article from Cornell University: Disease-Resistant Tomato Varieties.

Use of Fungicides

While cultural practices can help control foliar diseases of tomatoes, fungicides may also be needed. Apply fungicides, such as chlorothalonil, at regular intervals beginning 2 to 4 weeks after planting. Thorough coverage is essential. Be sure to spray both the upper and lower leaf surfaces as well as the centers of the plants. Spray to the point of runoff

Good luck, and may you beat it, or change things around such that you can wait it out.  Feel free to contact me if you need more info.