Recently, while researching steam treatment of weeds, I happened upon a marvelous paper called The Importance of Soil Fumigation: California Tree and Vine Crops.
Turns out that there is something called the “replant problem.” When a tree gets old (stone fruits last 15 to 20 years, grapevines last 20 to 35, and almonds last up to 25 years), they need to be removed. Since most commercial orchards follow a mono-culture agricultural model, growers want to quickly plant youngsters of the same variety. Often, this doesn’t turn out so well.
The reason new trees have trouble is that the soil is often infested with nematodes, small worm-like creatures that puncture holes in the roots and cause all kinds of damage, even to the point of starving the tree. New plantings placed in the same area will exhibit stunted growth and poor food production if there are already nematodes in the soil.
As if the nematodes weren’t bad enough, soilborne diseases from phytophthora fungi and bacterial canker will kill new trees.
To eradicate bad bacteria, fungi, and nematodes, growers use chemicals such as Chloropicrin, 1,2-dichloropropane (1,2-D), 1,3-Dichloropropene (1,3-D), and methyl bromide. ALL of these chemicals have been phased out by the EPA due to their carcinogenicity and toxic effects on humans.
So what are growers doing now? They cut their trees down to a stump, saturate that stump in Roundup, and starve the nematodes until they die. Then they plant a nematode-resistant rootstock like Nemaguard with their desired cultivar grafted on top. It’s not a great system.
The other way to handle the replant problem is through a ten-year fallow period, where soil organisms die off from not having the food they had gotten used to.
Apparently, the report says that steam doesn’t do the trick, even using 6” deep augers to apply the steam into the soil.
I’m not saying that there aren’t other answers; I’m just summarizing what this paper says. I’m leaving a lot of detail out, but if you’re interested, here is the link to the pdf: http://bit.ly/1vZeWOm Some of the statistics about the types of crops we grow in California and the percentage of that crop in the world’s overall production is impressive.
My suggestion is to stop monocropping. When an organism takes hold in a monocropped situation, it easily spreads from tree to tree. In a diverse ecosystem, even if a pathogen affects one tree, it’s unlikely to spread to dissimilar neighbors.
We know that diversity strengthens ecosystems, but our factory-farm mentality and “economies of scale” industrial models prevail in today’s fast-paced, profit-driven world. It’s only when nature gives us a wakeup call that we’ll have to bow down to the realities of this diversity issue.