New direction for replant research
A combination of seed meals shows promise as a control for replant disease.
Mark Mazzola checks a DNA sequence from streptomyces, a soil bacterium that induces plant defense responses. The growth of streptomyces is stimulated by seed meal amendments.
Fumigation has long been the standard treatment to combat replant disease in apple orchards, but studies by a U.S. Department of Agriculture researcher suggest that using seed meal as a soil amendment might give even better results in some situations.
This strategy would be compatible with organic production, and could provide an alternative to commercial fumigants, which might not be available in the future.
Dr. Mark Mazzola, with the USDA's Agricultural Research Service in Wenatchee, Washington, said there's been a reluctance to consider other methods of combating replant disease because of the complexity of the phenomenon. Chloropicrin has been used as a soil fumigant since just after World War 1; methyl bromide and Telone (1,3-dichloropropene) have been examined, used, and studied since the 1940s; and Vapam (metam sodium methyldithiocarb) has been used and studied since the 1950s. And there's still research being conducted to try to optimize how those fumigants work.
Mazzola has been researching replant disease for 13 years, but has taken a different tack, looking for alternatives to fumigants. After examining the soil in many orchards in central Washington, he can say with some certainty that the replant phenomenon is caused by a specific complex of organisms in the soil: the fungi cylindrocarpon, phytophthora, pythium, and rhizoctonia, along with the lesion nematode (Pratylenchus) in some sites. Although the proportions of those various organisms and the species vary from site to site, as long as they are all targeted, replant disease can be controlled, he believes.
In a research trial planted in 2002 north of Wenatchee, he tested a canola seed meal soil amendment, applying it in three different ways: as a soil amendment covered with plastic to cause solarization; as a soil amendment after a wheat cover crop had been grown for four years; and as an amendment combined with a postplanting drench of the fungicide Ridomil (metalaxyl).
Judging by tree growth measurements during the first four seasons, the seed meal and Ridomil drench performed as well as fumigation with Telone C-17 (Telone with 17% chloropicrin). The reason the other canola treatments were not successful was because they failed to control one or more of the pathogens that cause replant disease, Mazzola said. For example, the solarization treatment provided total control of rhizoctonia but failed to control pythium or cylindrocarpon. In fact, it stimulated pythium in the soil. By combining the seed meal with Ridomil, pythium was controlled. There were no lesion nematodes at that site.
He has found that during the initial phase of orchard establishment, seed meal treatments can perform better than fumigation, in terms of tree growth, even though the disease control by those alternative treatments is less effective, he said.
"I think that suggests that the fumigation treatment is impacting something with regards to the overall soil biology in a negative sense," he said. "It suggests that the grower's missing out on something the first year or two of tree growth."
Mazzola has also been testing other seed meals, including Sinapis alba (white or yellow mustard) and Brassica juncea (India or oriental mustard). Tests were done with Gala trees on Malling 26 rootstocks. India mustard, when applied alone, provided little control of replant disease, judging by tree growth after three seasons and by early yields. However, when combined with Ridomil, it performed better than fumigation. To find out why the seed meal did not work alone, Mazzola examined tree roots and by isolating the DNA discovered that they were all infected with phytophthora.
He was disappointed the seed meal could not be used alone, as he had hoped to develop a treatment that would be useful in an organic setting as well as conventional orchards.
Mazzola went on to test formulations of multiple seed meals, based on what he has learned about the activity of each of them against the pathogen complex that causes replant disease.
Preliminary tests show that a combination of canola and mustard might provide better control of pythium than Telone C-17, and also good control of lesion nematodes. In a trial of Gala planted in May 2007 at the organic orchard of Ray Fuller in Chelan, Washington, tree diameter after the first growing season was the same for the seed-meal-treated trees as for the fumigated ones, which were both significantly greater than untreated.
He believes that rootstocks with some tolerance to the replant pathogens are likely to be part of the solution. For most of his trials, Mazzola has been using trees on the M.26 rootstock, which is highly susceptible to replant disease.
He tested a range of rootstocks for their tolerance to pythium and lesion nematodes, and found Geneva 11 and G.30 to be tolerant of both, while M.26, M.106, and M.111 were highly susceptible to both. Malling 9 proved fairly tolerant of pythium but susceptible to lesion nematodes.
Mazzola said he would expect even better results with the seed-meal treatments if they were used in combination with a tolerant rootstock, rather than M.26. "I think the upper end of the disease control we can expect has not been anywhere near reached or obtained."
In rootstock trials, growth of trees on the G.11 and G.30 rootstocks has been variable from site to site, he noted, but research has not yet focused on the reasons for the variations. "You have to ask why, and that question has not been answered," Mazzola said. "I think the evaluation needs another layer to find out why something is not performing."
Mazzola's work on replant disease is no longer being funded by the Washington Tree Fruit Research Commission, but he recently received a competitive grant from the USDA for more than $500,000 that will take his work in a different direction, he said. He will study the modes of action of the seed meal amendments and take a closer look at the very complex biological systems in the soil.
In order to achieve predictable results, the mode of action must be understood, and this involves monitoring microbial communities over time, he explained.
"We're looking at the broader spectrum of the soil biology. A lot of the work will be done at the molecular level so we can get a better picture of what's going on."
The various seed meals do not work in the same way, he said. India mustard generates a natural fungicide called allyl isothiocyanate (AITC), which controls pythium. AITC is a volatile that only remains in the soil for one or two days, but it appears to change the biology of the soil dramatically in a manner that leads to disease control. Although the pythium organism is still in the soil after the AITC is gone, it no longer causes disease. "We think it's other elements of the soil biology that are not enabling the disease to occur," he explained.
He will also study the impact of seed meals on the microbial community involved in nitrogen cycling, as the nitrogen content of seed meals is 6 percent. He expects the different seed meals to have very different impacts. "We know that AITC wipes out the free-living nematode community, which is involved in nitrogen cycling, so is that going to negatively impact nitrogen availability? And if so, for how long? And what impact is that going to have on tree growth?"
Mazzola said scientists in other parts of the world, including South Africa, Poland, and Switzerland, have been interested in his research, and he has been invited to go to a Swiss research station to help researchers set up a program on replant disease.