Ruud Kleinpaste
The plant diseases to watch out for
Black spot is one of those diseases that turn up every year, but the infection starts earlier than you think
We instinctively know that hot, humid conditions are perfect for fungal growth and bacterial division, because we see the effects mostly in the warm summer months. But Nature has evolved organisms for every available weather condition and that includes sun, shade, wind or shelter, warmth, humidity and indeed: cold, driving sleet and rain.
Powdery mildews love hot autumn days and cool nights. This causes dew, which is perfect for germinating spores. Think pumpkins, oaks, hydrangeas, apples, roses and grapes. And as soon as the fungus is established on the leaves and buds, it can continue to grow, even when conditions become dry.
Powdery mildews are white spots and associated with warmer times. What about the black spots that seem to turn up in the cooler conditions of late winter and early spring?
One that rosarians know all too well is Diplocarpon rosae or rose black spot. It’s one of those diseases that turns up every year, but the infection starts much earlier than you think.
The spores overwinter on pruned twigs and buds as well as fallen leaves, dumped below the bush. Hygiene is crucial to reduce infection rates: good, open pruning allows lots of drying air movement around the plant. A sunny position without under-planting will keep the leaves and stems mostly dry, especially if you only water the roots.
The idea is to keep the plant healthy and in good condition through the warmer months of the year, when growth and infection rates are higher.
The odd copper spray will protect the new leaves from spore invasions; this could be handy to know when the meteorologist tells you there’s a heavy downpour on the way.
But remember: get rid of the fallen, infected rose leaves!
If you can manage such a disease threat, you should be able to grow a decent apple too – apple black spot or black scab, Venturia inaequalis, is another type of fungus that loves the warmer weather. It can spread quickly, but the infection tends to begin in early spring. What gets the fungus off the ground and onto the apple tree framework and buds are wet, squally showers; the really cool south-westerly fronts that chill you to the bone.
Venturia is unmistakable: small, dark spots on new leaves, perhaps a few millimetres in diameter. This is followed by black spots on developing fruit, which become quite misshapen.
A meteorological forecasting model helps commercial apple growers to calculate the risk of infection. I reckon gardeners can learn from it too – that is, cover your trees with some fungicide before a cold front hits!
Bacterial blast on stone fruit ( Pseudomonas syringae) is another bother, albeit a gummy-oozy one. Also called gummosis and bacterial canker, its main hosts are apricot and cherries (especially young plants), but other summer fruit are susceptible too.
The blast bacteria are basically everywhere and arrive on the target plants via cold, wind-driven rain in spring, through insect movement and infected pruning equipment or plant stock. As ever, careful pruning and hygiene is vitally important.
This species has been found to produce proteins that are biological ice nucleators, which means not only can they cause freezing of the water inside and outside plant tissues, they also play a part in the formation of rain, sleet and snowflakes!
As soon as ice crystals are formed and expand, they cause internal plant damage, allowing the Pseudomonas to invade. It’s clear that infection rates are high in winter and early spring, especially when trees are damaged and have open wounds. Another reason to protect your trees right now!
Actively growing trees are not usually infected, however, which makes some folk recommend pruning in autumn before leaf-fall.