by Michelle Cortens
The tree fruit industry in Nova Scotia relies on the healthy re-establishment of trees on old orchard sites for continued success. “Out with the old and in with the new” has been a revolutionary trend with the release of exciting varieties such as Honeycrisp and Ambrosia, dwarfing rootstocks, and high-density planting systems.
In many cases, old orchards are being replaced with new orchards on the same land – in what is known as a replant site. However, replant sites often have limitations. They harbour the pests that previously fed on the roots of established apple trees. Not surprisingly, the pests are eager to prey on new trees.
The symptoms of Apple replant disease include trees with stunted and uneven growth throughout rows, low yield, and occasionally death. Affected trees will often take an extra two or three years to become fruitful because of slow establishment, which impacts the economic viability of the new orchard.
The pressure of Apple replant disease can severely restrict the health and productivity of new plantings. Not long after planting, trees begin to show symptoms if they are affected. Trees struggle to grow because their small and decaying root systems harbour the pathogens and parasites associated with Apple replant disease. The causal organisms in Nova Scotia are the pathogens Pythium, Phytophthora, Rhizoctonia, and Cylindrocarpon, and the parasite Root lesion nematode (Pratylenchus penetrans).
The pathogens are commonly present in soil systems, and can persist in soil for many years in protective structures. Periods of sustained soil moisture can activate and distribute these pathogens. Poor growth in apple trees can be related to the infection of young tissue such as root tips, leading to the release of systemic toxins and growth inhibitors.
The Root lesion nematode is the most economically important parasitic nematode for apple trees in the Annapolis Valley. It’s a worm-like organism that is invisible to the naked eye and lives in soil and plant root tissues. Nematodes become active when the soil warms up in spring, live inside roots especially during hot, dry summers, and continue to be active in the fall before the soil freezes. When the root is depleted of nutrients, the nematodes begin feeding on a neighbouring tree. The most noticeable symptom of nematode damage in orchards is a wave-like distribution of uneven tree growth. At the trough of the wave, severely stunted trees are parasitized by high populations of nematodes, while relatively taller trees at the crest of the wave have fewer nematodes.
Management targeted at Apple replant disease can help minimize its risk and severity. The emphasis is on prevention and pre-plant management because limited practices are available to manage replant problems after plants are established.
First, the risk should be diagnosed pre-plant by identifying if tree fruit have grown on a particular site within the last 30 years. Sample for nematode population density by collecting soil samples using accepted sampling methods. The economic threshold for nematode populations in apple orchards is 500-1,000 Root lesion nematodes per kilogram of soil.
Second, consider using a combination of land preparation practices that are effective and reliable. Practices include removing leftover roots from orchard crops, planting specific cover crops that suppress nematodes, soil fumigation, and applying amendments such as compost or mulch.
Finally, plant the new tree row in the alleyway of the former orchard – where pest populations in the soil are lowest. Choose a ground cover between orchard rows that does not host nematodes. Examples include Creeping red fescue and Perennial ryegrass. Take note that even though some rootstocks are rated as “replant-tolerant,” they are not tolerant of nematodes.
In theory, a soil that is suppressive to Apple replant disease could be produced by encouraging populations of beneficial soil organisms that prey on pests. Such a long-term strategy may improve tree establishment and enhance production on replant sites by keeping pest populations in check. However, more needs to be learned about the complex soil biology in orchard replant soils in order to create disease-suppressive soils.
If trees show symptoms of Apple replant disease after they have been planted, contact your local extension specialist for assistance.
(Michelle Cortens is a tree fruit specialist with Perennia Food and Agriculture Inc. based in Kentville, N.S.)