John Scrace was the first speaker of 2023 at the January meeting of Ticknall Garden Club. He came with in-depth knowledge of pests and diseases that can plague the plants in our gardens. He is a freelance pathologist whose advice can be readily accessed on the Royal Horticultural Society website. www.rhs.org.uk
He started with the all-too-familiar red lily beetle which was around in the 1940s but in succeeding years has multiplied at an alarming rate. Eggs on the underside of leaves hatch out to black larvae which do the most damage. He advised removing by hand. Putting paper under the plant first will make it easier to catch the adult red beetle. Apparently, they make a cute squeaking alarm call when caught! Alternatively, an insecticide can be used, or calcium chloride painted on leaves.
An infestation of viburnum beetle can destroy a bush leaving only skeleton leaf shapes. The larvae and adult beetles do equal damage. An insecticide or biological control with nematode worms can be used but sometimes replacing the bush with an alternative is the only answer.
The vine weevil is the scourge of plants in containers and can remain undetected until the plant withers with its roots eaten away. The white grub hides unseen in the soil. The adult beetle (all female) nibbles the edges of leaves at night. A vine weevil insecticide is available as well as traps with biological controls.
A new pest has emerged in recent years in the form of the harlequin ladybird. It was first used in glass houses in Belgium to control, very successfully, aphid infestation. Escape into the outside world and its speedy spread throughout this country has proved more problematic. While certainly useful for feeding on aphids it unfortunately turns to our native ladybirds as well as other useful insects when there are no aphids around. They can be identified by a triangular white spot on their heads, orange legs and brown undersides. They also tend to hibernate in large numbers inside buildings. The native 7 spot ladybird is still thriving and it is assumed that eventually a natural balance will develop.
A spell of warm and wet weather encourages potato and tomato blight. Spores, spread by wind and rain, damage stems and leaves which turn black and damage tubers in the ground. No control is available. Planting early potatoes and blight resistant tomatoes was advised.
In recent years there have been more diseases affecting trees. A familiar sight early in the growing season has been the leaves on horse chestnut trees turning brown. It is caused by a leaf miner whose caterpillars infest the tree in their thousands. Birds and parasitic wasps will eat some, but no other remedy exists. The same tree can also suffer from bleeding canker which can kill the tree. In general the only hope for survival in threatened trees is the discovery of disease resistant varieties.
It was not surprising to hear that the number one pest for enquiries to the RHS was the slug.
It is the tiny black slug that does most damage. The big slugs in fact eat them and also help with composting. The familiar methods of treatment were covered but in the long run planting slug resistant plants was the only foolproof solution.
The number one disease was the honey fungus. The white “bootlace” threads on the roots of trees and bushes creep out to infect other roots causing slow death. If white fungal growth is seen when bark is scraped away the honey fungus is confirmed. There is no treatment apart from digging out the root system thoroughly. There are some plants that are resistant to the disease.
John Scrace, with the benefit of his scientific background, gave his audience a comprehensive guide to treating just a few of the many problems that can affect the plants grown in our gardens.