What is Root Rot?
Root rots and crown rots are caused by multiple fungal species from the genus Phytophthora (P. cactorum, P. syringae, P. cambivora, P. citricola). The specific species of Phytophthora vary from region to region, and the symptoms vary a fair amount across species. Generally, a Phytophthora root infection will destroy and decay tissue in the roots, leading to a loss of nutrient/water acquisition and eventually a visible tree-wide decline. The decline can be rapid throughout the course of a season or slowly occur over many years, depending on the severity of the infection. This disease is challenging to diagnose as the early signs are in non-visible tissue and symptoms can be analogous with other biotic and abiotic stresses (e.g., overly wet soil).
What does Root Rot look like?
The first noticeable above-ground symptoms include growth stunting, wilt, off-color foliage and general decline. These symptoms can resemble a multitude of other issues both disease related and abiotic (e.g. nutrient deficiency, cold injury, rootstock fire blight). Symptoms can persist and worsen over multiple seasons as the fungus overwinters and re-establishes the next spring. To assess whether an infection is due to root rot, dig out the soil around the base of the tree and cut into the wood to observe any discoloration. Infected wood will have a reddish-brown discoloration in the cambium layer. Phytophthora can also affect the base of the trunk when discoloration can be seen on the wood at the soil line, indicating a crown rot. This trunk damage can be confused for cold injury or fire blight damage in the rootstock.
Where does Root Rot come from?
Phytophthora species responsible for root rot are often naturally present in agricultural soils, looking for new hosts to infect. For Phytophthora-free soils, introduction of the fungus is possible via contaminated soil amendments, tree stocks or irrigation. Higher rates of root rot infection occur during seasons with heavy rainfall in the early spring. Heavily saturated soils often subject trees to sit in standing water, an ideal environment for Phytophthora development. Wet soils also deprive the roots of oxygen, making the tree more susceptible to infection. Over-saturated soil is the only environment in which noticeable and significant progress will be made by the fungus to establish and digest root tissue. Throughout the season, spores will be released into standing water in the soil, further spreading disease. At the end of the season, Phytophthora will overwinter either as spores or full fungal structures sheltered within the root tissue. Winter temperatures slow fungal activity, and the following spring rains will determine the number of spores and fungal colonies that continue infecting the orchard.
How do I prevent and control Root Rot?
– Avoid low spots in the orchard that frequently flood
– Provide additional drainage to the trees
– Install drain tiles and diversion ditches
– Rip underlying pan layers
– Plant on ridges or berms to raise the level of the crown
– Mitigate soil compaction
– Plant Phytophthora tolerant Geneva rootstocks (e.g., G.11, G.16, G.30, etc)1
For more information on Crown Rot: