Autumn presents a magnificent opportunity to enjoy the garden thanks to the vibrant foliage, however gardeners must remain vigilant against black spot disease which could prevent roses from blooming ever again. As temperatures drop and rainfall increases, black spot becomes more prone to spreading throughout gardens – a fungal infection that initially manifests as large dark circular marks on yellowing rose petals.
Cathy, a horticulturist and co-founder of The Wieders Garden, revealed that whilst black spot rarely proves fatal to roses, it renders the blooms extremely susceptible to harsh weather conditions, garden pests and additional ailments. She explained: “Blackspot is not usually lethal in and of itself, but it can severely weaken a rose to the point where it won’t survive unfavourable weather (protracted drought conditions, for example) or a very harsh winter. Since it affects the leaves, plants that struggle to produce adequate food can’t produce the same number of buds and those it does produce are often smaller and not the best form.”
Once black spot disease establishes itself on a rose bush, it proves exceptionally difficult to eliminate, though fortunately it remains remarkably straightforward to prevent using one organic solution.
Cracked corn, commonly discovered in bird feeders or used for feeding poultry, is frequently utilised by gardeners as a soil enhancer to ward off fungal infections. Cathy explained: “Cracked corn is an inexpensive source of the beneficial fungus Trichoderma which research has shown to be an effective fungal biopesticide.
“We can attest to its efficacy. We have been treating our rosebeds with cracked corn since 2005 and have noticed a dramatic decrease in the incidence of blackspot when we put it down in early spring compared with those years when we either didn’t use it at all or put it down much later in the season.”
Black spot can only spread onto rose foliage through moisture, but cracked corn aids in improving soil drainage, reducing humidity around plants and minimising the chances of rainwater splashing onto leaves.
Cracked corn can be sourced from certain gardening centres, but it is also readily available at many online stores such as Amazon or eBay.
The process is simple: just sprinkle a generous amount of cracked corn on rose beds before mulching them or covering them in compost to help keep roses healthy.
However, the simplest way to ward off black spot is to maintain a tidy garden this autumn and ensure any decaying autumn leaves are promptly picked up.
Cathy stated: “Cleaning up any damaged or infected leaf litter goes a long way toward helping keep fungal infections like blackspot from spreading through a garden.”
Infection can also be minimised by ensuring roses are properly cared for through weeding, watering and feeding them when necessary, but you can also opt for rose species that are less likely to be susceptible to the disease.
Cathy stated: “Choosing varieties of roses that are known to be naturally resistant to blackspot can also help, but resistant does not mean immune and even the highly resistant Knock-outs can develop blackspot in the “right” (or wrong) circumstances.”