by Bill and Mary Weaver
Your worst nightmare as a Christmas tree grower could become a dream come true for the right researcher. Don’t suffer in silence. Bend the ear of a researcher. You and a lot of other growers with similar problems, might be glad you did.
A Connecticut Christmas tree grower had a “nightmare” field. Every native fir he planted in it died. Fortunately, he mentioned his problem to researcher Dr. Richard Cowles. Cowles took a look at the field. “I’d been asked a year before the CoFirGE trials [Cooperative Fir Germplasm Evaluation project] started to work on this problem by the Connecticut Christmas Tree Growers Association (CCTGA),” he explained. “It’s unusual to find a field so reliably infested, wall to wall, with phytophthora. We did a test planting of native (Fraser and Canaan) firs, along with Turkish and Nordmann firs. At the end of 4 years all the Fraser fir were dead and the Canaan fir were suffering from the symptoms of soon-to-be-terminal Phytophthora root rot.” Following that trial, Cowles viewed the field as pure gold, as it provided a research opportunity to test ideas for protecting trees from Phytophthora.
It’s still early in the game, but so far, some of his theories appear, as he puts it, “to have legs.” One factor being tested is to turn on the trees’ ability to protect themselves. The other idea is to drastically lower the pH of the soil, to make conditions more favorable for the tree than for the disease.
Tom Rathier, a retired soil scientist and the current president of the CCTGA, had experimented with this in a well-drained field with no Phytophthora problems, just to see how low a pH fir trees could tolerate. Using sulfur, he took the soil pH down to an extremely acidic pH of 2, and the trees grew fine.
“This makes intuitive sense. Where do fir trees grow naturally? On mountain tops with thin, acid soil.” Fraser firs in North Carolina naturally grow in mountain soils with a pH that can be around 3.5. “The worst Phytophthora problems I’ve seen have been in fir trees planted in fields that had previously been planted in corn. These fields are typically limed to a pH of 6.
“Zoospores produced by Phytophthora have flagellae and swim through the water-filled soil pores to reach new trees to infect. Conveniently, zoospores are NOT tolerant of highly acid soils.” Some other factors are at play at a very low pH that could benefit the trees. “A general rule of thumb is that the lower the pH, the fewer the organisms that can survive in that soil. Low pH may limit activity of nematodes that open wounds in roots, which may encourage infection by root rot organisms.”
To Cowles, it all made sense. He’d just needed a chance to do the experiments to test his hypothesis. “By lowering the soil pH with sulfur, we have the opportunity to create the conditions in which the fir trees will grow and thrive, but the Phytophthora cannot.” The nightmare field gave him that opportunity. Using this “gold mine” of an infested field, he didn’t need to inoculate his trees with Phytophthora as researchers in other areas did. The wall-to-wall Phytophthora in the field took care of that.
Instead, his first step the year before planting was to add sulfur to the soil to lower the pH from its current nearly neutral pH of 6.2 to a very acidic pH of 4.
He used pelletized sulfur. “Even with pelletizing, the sulfur dust gets loose and is a strong irritant to skin and eyes, and is difficult to launder out of clothing. The amount of sulfur needed is determined by the soil type and texture. I relied on the expertise of Tom Rathier to help me with that.”
After the sulfur was applied, it was incorporated into the soil with a rototiller so it would reach the root zone. The sulfur had 6 months, including the winter, to break down and activate in the soil before the trees were planted. “At planting, we could still see bits of sulfur, so we may not have seen its full effect yet when we planted the fir trees. However, the soil now has a pH of 4.
“We are comparing Fraser firs, which have no genetic resistance to Phytophthora, and Canaan firs, which have some resistance. We also tested a combination of root dips that we hoped might turn on the trees’ self-protective mechanisms to fight Phytophthora infection. Along with the sulfur treatments to lower pH there are a total of 28 treatment combinations in 23 replicates, 644 trees in all.
“We’re taking measurements on each tree. With a factorial design experiment like this, you can tease apart the various factors and determine very quickly what sorts of changes are making a significant difference.
”After rating these trees in late summer, the significant statistical results practically knocked me over. I know it’s early in the experiment. I should only crow when the trees are 6 feet tall and salable in this ‘impossible’ field. But it is already clear that drastically lowering the soil pH by applying and incorporating sulfur is making a tremendous difference in the health of both Fraser and Canaan firs in their first year following planting, and this improvement is consistent with reduced root disease.”
Also, the root dip with potassium phosphite benefitted the very susceptible Fraser firs, but not the less susceptible Canaan firs in the first year. If all this holds up over time, it may be that the Fraser firs will benefit later from a basal bark spray with the same material, to continue turning on the plants’ immune response.
In an already-planted field, a surface-only application of sulfur to lower the pH may take 3-4 years to show a difference, but Cowles and Rathier have already seen such a difference in a field of Fraser fir that was suffering from interior shoot discoloration. “You see this frequently with Fraser fir. Most growers will be quite familiar with the syndrome.
“I call it ‘poor root function syndrome.’ There’s great shoot growth on the outside of the tree, but if you go even slightly inside the tree, the less dominant terminal shoots look off-color – even white or brown. The only solution used to be cutting down the trees and throwing them away.
“We tested the soil around each tree, and found that the lower the pH, the better the tree. Based on this observation, the grower applied pelleted sulfur to the soil surface. Three to four years later, the trees are now gorgeous and salable. By improving the health of the root systems by lowering the pH, we improved the health of the whole tree.”
Is pH the key to the puzzle?
by Bill and Mary Weaver