The good news is that black spot is unlikely to kill your rose plants, but it can significantly affect the way the plant looks, and it does weaken it making it potentially more vulnerable to pests, and other diseases – which can go on to cause further damage.
A black spot prevention strategy relates to managing the environment to try and prevent the fungal spores from germinating in the first place. There are four core pillars: watering, air management, hygiene, and nutrition.
Watering
- Apply water at the base of the plant, never over the foliage. This is because fungal spores love wet leaves so water on foliage needs to be avoided (unless you are applying a foliar fertiliser – more on that later).
- Apply water in the morning rather than the afternoon or evening. This allows accidental splashes to dry in the sun.
Air Management
- If possible, space roses well apart and prune them to create an open centre (goblet shape). This allows wind to dry the leaves faster after rain.
- Be aware that if you are training climbing or rambling roses against a wall, it is likely to be more difficult to control black spot because air flow will be reduced.
Hygiene
- Fungal spores can travel on your secateurs. Wipe blades with a garden disinfectant or surgical spirit between pruning different bushes.
- In autumn/winter, clear away every single fallen leaf from around the base. Do not compost them; put them in the council green bin or burn them, as spores overwinter in the soil and debris.
Nutrition
- Apply a targeted rose fertiliser to the soil around roses in early spring to give them a strong start. Nutrigrow Rose Fertiliser contains 5% nitrogen, 5% phosphate and 10% potassium. The high level of potassium is ideal for roses as this macronutrient is vital for the formation of chlorophyll and the production of blooms.
- Apply a thick layer (5–10cm) of well-rotted manure or compost in late winter after clearing old leaves. This will seal any remaining spores in the soil, preventing them from splashing back up onto new spring growth.
- Use a balanced foliar fertiliser from spring onwards starting with an application to young leaves before buds appear. Continue to use this foliar nutrition according to the rate shown on the label every 2-3 weeks in the growing period. Apply it in the morning. Nutrigem 15-15-15 contains an equal amount of the macro nutrients nitrogen, phosphate and potassium, together with seaweed, magnesium, iron, zinc, copper, manganese and boron. This nutritional powerhouse provides superior stress relief and promotes vigorous plant health to support the rose’s ability to fend off fungal infections.
- SB Plant Invigorator can be used throughout the year to boost plant health and can be used alongside a foliar fertiliser. It contains a unique blend of plant safe, physical pest control surfactants to provide a non-chemical mode of action, helping to suppress fungal diseases. It does this by physically disrupting fungal spores on the leaf surface making it harder for them to gain hold. It is safe for birds and bees.
Finally, if you are planting new roses, look for the RHS Award of Garden Merit (AGM) or the ADR (German) label, which signifies high disease resistance. 'Siluetta' (climber), 'Lucky!' (floribunda), or 'Silver Anniversary' are varieties known to be more resistant to black spot.
This post was written in collaboration with Harry Glenn.
Harry Glenn MBPR FQA
Harry Glenn is Agrigem’s Technical Account Manager in charge of sports and horticulture. Prior to his move to Agrigem, Harry enjoyed a career in business development with, among others, a well-known sports turf contractor. This role exposed him to some of the UK’s most prestigious sporting venues including Cheltenham Racecourse, Wimbledon, Wentworth and Lords - delivering a lifelong appreciation of the benefits of high-quality sports turf.
An experienced Account Manager, Harry has a genuine interest in the sports turf sector and underpins much of his advice and guidance with practical experience at the coalface of turf growing.
Article Notes:
Nutrigrow is a trademark of Agrigem Ltd.
SB Plant Invigorator of the Stan Brouard Group.
Westland is a trademark of Westland Garden Health.
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