The rainy spring weather in our climate makes prune plums susceptible to disease.
Dear Helen:We have four Italian prune plum trees in our garden, already mature when we bought the house 22 years ago. The plums are delicious although racoons knock most of them down before they can fully ripen. It is usual for small ones to fall now and again, but this morning a visitor noticed some on the ground looking purple-brown and shrivelled. They look like tiny prunes. Should we be worried?
A.A.
The rainy spring weather in our climate makes prune plums susceptible to disease. A major one is brown rot, which begins with browning flower petals and continues with soft spots on the fruit that lead to rotting. Infected plum clusters appear as a glued-together mass of shrivelled, mummified fruit on the tree. Infected individual plums shrivel and drop from the tree.
Another disease is black knot, whose main identifying feature is the appearance of swollen, rough swellings (the “knots”) on twigs and branches. They start out brown and turn black as they enlarge. Black knot does not usually affect the fruit itself, but it can weaken trees enough to affect fruit production.
As you have noted, it is normal for a plum tree to drop excess fruit, but your description of the browned and shrivelled, prune-like fallen plums sounds suspiciously like brown rot.
Because prune plum trees are prone to disease infections, they need to be checked regularly for infected fruit and twigs and branches that do not look normal. Some suggestions:
Remove any “knots” on a tree, making cuts 10 to 15 cm below them. Do not compost removed material. Similarly remove mummified fruit clusters, and pick up and dispose of all fallen fruit.
Keep the trees pruned (in mild, dry weather) and well thinned to create an open-centred shape that allows all parts of the tree to have optimum exposure to sunlight and fresh air. Thin the fruit enough to allow each remaining plum to stay dry and unbruised. Water at ground level to keep water off the tree canopy as much as possible. Avoid over-fertilizing with nitrogen, which promotes lush, potentially disease-prone growth.
As for raccoons, I tried many tactics to keep the clever creatures out of my tree, including battery powered, high-frequency sound emitters meant to deter them — until a mother raccoon parked her three babies in the tree. I presume the babies had not yet acquired full hearing.
The easy deterrent that I eventually came upon was a piece of sheet metal wrapped and secured around the tree trunk. Even wily raccoons cannot climb up sheet metal. This simple device works as long as there is no other access to the plum trees from nearby structures or other trees.
Dear Helen:Five years ago I planted a Japanese snowbell (Styrax japonicus) tree, which has grown beautifully and flowered well. Last year, every branch was outlined in flowers and there was a heavy crop of fruits. This year, the tree looks very healthy, but there were very few flowers. I am wondering if the February 2023 and 2024 cold snaps resulted in “mast years” and the tree just put less energy into flowers this year as a result. What do you think?
L.W.
It is, apparently, a not uncommon evolutionary strategy of trees to alternate a year, when conditions are optimal for it, of abundant reproductive growth (fruit and seed production to ensure survival of the tree’s species) with a year of mainly vegetative growth. Home gardeners often see this cycle in their apple trees.
Trees sense and allocate available resources (water, sunlight, nutrients) to their best uses. Resources at their most abundant tend to be funnelled into species survival mode in seed production.
Years of abundant fruit and seed production are called “mast” years. Reasons why they occur are not fully understood, but as you suggest they can also be prompted by environmental factors. Though Japanese snowbell is hardy in our climate, unusually cold temperatures, especially very late in winter when plants are particularly vulnerable to extreme cold as they begin emerging from dormancy, could possibly affect fruit and seed production in some trees.
Garden advice.The Victoria Master Gardener Association offers a free Ask a Master Gardener service featuring a team of master gardeners who will respond with well researched answers to questions from the public that are submitted via e-mail to[email protected].
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