Before replanting an orchard, apple growers are generally advised to fumigate the soil to prevent replant disease.
Replant disease is caused by a build-up of microorganisms in the root zone of the trees, which negatively affects the growth and productivity of new trees planted into the same soil.
But fumigation with a chemical, such as Telone C-17 (1,3 dichloropropene and chloropicrin) only provides short-lived control of harmful soil organisms, says U.S. Department of Agriculture plant pathologist Dr. Mark Mazzola. He has identified an alternative treatment that has longer lasting effects and even greater benefits than fumigation in terms of both tree growth and fruit yields.
He said growers probably aren’t aware that trees in fumigated soil are still underperforming, even though they do better than trees with no replant disease control. That’s because until now growers have had nothing else to compare to fumigation.
Mazzola has found that a combination of yellow and white mustard seed meals incorporated into the soil as a preplant treatment controls all the organisms implicated in replant disease in Washington State orchards, including nematodes. And the treatment could be used by organic growers. Many currently drop their organic status while replanting their orchards
so that they can chemically fumigate the ground to avoid having an underperforming orchard.
Mazzola came to the Agricultural Research Laboratory in Wenatchee, Washington, in 1995 specifically to work on control of replant disease.
His first task was to identify the causes. The main culprits turned out to be the fungal organisms Pythium, Rhizoctonia, Ilyonectria, and Phytophthora, along with the lesion nematode. The relative importance of the various organisms varies from site to site.
Mazzola first tested the idea of growing a cover crop in the soil for a year before replanting, with the idea that this would change the soil microbiology. He tried a variety of crops, such as wheat, canola, and Sudan grass, but none provided the comprehensive benefits he was looking for. Leaving the ground fallow for as long as three years resulted in “not one iota of change in disease development,” he reported.
He then turned his attention to seed meal, which is a byproduct of biodiesel production. He tried canola, rape, white mustard, and yellow mustard seed meals, but both greenhouse and field trials showed that none of them used alone controlled all the replant organisms. For example, canola, which gave good control of Rhizoctonia, increased Pythium.
He then tested various combinations and ratios of the seed meals. Based on cost and efficacy, the winning formulation proved to be a half-and-half mix of yellow mustard (Brassica juncea) and white mustard (Sinapis alba). The best time to apply the seed meal is in the fall before a spring planting.
The mixture appears to have both chemical and biological modes of action. The seed meals produce chemicals that, through an enzymatic reaction, can be toxic to the pathogens, but they also change the microbiology in the soil to make it more resistant to reinfestation by disease organisms in the long term.
Mazzola reported that in a field trial planted in 2010, Jonagold trees on Geneva 11 rootstocks yielded about 11 kilos per tree (24.2 pounds) in the first two cropping years in both the control and fumigation treatments. Trees with the seed meal amendment produced 16 kilos per tree (35.2 pounds), a 45 percent increase in yield.
A Gala planting on Malling 9 and G.11 rootstocks, also planted in 2010, showed similar trends, with cumulative yields from the first two crops 25 percent higher in the seed meal treatment than in the fumigated plots.
When Mazzola looked at lesion nematode populations, he found that both the fumigation and seed meal treatments suppressed them initially. While fumigation might kill harmful, plant-parasitic nematodes in the tree root zone, they are found throughout the soil profile and can migrate back into the root zone.
“The real story comes after the first year,” he said. “At the end of the second growing season in the fumigated soil, the numbers are higher than the control. Those in the seed meal treatment are significantly lower.”
The suppressive effect of the seed meal treatment could still be seen after the fourth season.
It was a similar story with Pythium. Both fumigation and the seed meal treatment reduced the rate of infection in the first year, but by the end of the second season, Pythium had come back into the fumigated soil.
Mazzola and his colleagues used a technique called metagenome analysis to identify the microorganisms in the rhizosphere (including the roots and the attached soil) and find out what had changed after the fumigation or seed meal treatments. This involved generating and sifting through millions of DNA sequences. He believes he is the first to use this powerful technique to study orchard soil microbes.
The analyses showed that after two growing seasons the microbial community in fumigated soil had reverted right back to that in the control soil. In contrast, the microbes in the seed meal treatment were distinct and included a number of organisms that suppress disease.
For instance, Arthrobotrys, a fungus that attacks nematodes, and Dactylella oviparasitica, a fungus that parasitizes nematode eggs, were found only in the seed meal-amended soil. These nematode parasites were likely responsible for the long-term suppression of lesion nematode in the seed meal treatment. Another fungus, Oidiodendron, which is known to control Phytophthora was also only found in the seed meal treatment.
Mazzola, who is a forest biologist as well as pathologist, said the soil after a preplant treatment is a little like a forest after a fire. Organisms move back into the soil in
succession over time.
The succession in the fumigated plots leads back to where it started, whereas the changes in microorganisms in the seed meal–treated soil are longer lasting, he said. “We’ve altered the system in a means that does not revert back to the control. It seems to promote organisms that make the system more resilient to pathogen invasion.”
Mazzola said the dogma is that a more diverse system is a more resilient system, but he’s found that the opposite is true in this instance. The metagenome analysis showed that after the combined mustard treatment the microbial community was significantly less diverse than in fumigated soil, yet it was more resistant to pathogen invasion.
“It’s not necessarily the diversity that leads to a more stable system, but who’s there and who are the players,” he said.
His research also showed that the mustard seed meal treatment can provide weed control. In addition, the metagenome analyses revealed higher populations of bacteria that are able to metabolize potentially toxic organic compounds. This suggests that the seed meal might promote degradation of pesticides applied to the orchard, though this would have to be investigated, Mazzola said.
The next step in his research is to find out if he can reduce the rates of seed meal needed by using a rootstock that has some tolerance to replant disease, such as the Geneva 41 and G.210. So far, he’s been applying three tons per acre. He will test rates as low as half a ton.
He’s also looking at ways to enhance the activity of soil microorganisms in order to further promote tree growth. “We know there’s a whole host of organisms in the soil that are growth promoters or nitrogen fixers,” he said.
He’s also looking at various fertility inputs, such as compost, applied after the orchard is planted, to see if they can help diminish lesion nematodes. •