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. •