leaves on Rhododendrons
rhodos have developed a yellowing leaf with patches of ginger staining
on the underside. How do I treat them?
your description, it sounds like your rhododendron has rust. Rust
is a fungal disease that is common in periods of wet weather. Once
rust has set in, it can be hard to control. Prune out infected shoots.
Clean up any fallen leaves around the plant and destroy by burning
or placing in the rubbish bin (not the compost heap). Spray with
an appropriate fungicide such as Fungus Fighter a broad spectrum
systemic fungicide. Systemic sprays are absorbed by the plant and
carried in the sap giving better control. Spray at 7-10 day intervals
for complete control. A maximum of 4 applications per growing season
is recommended as disease resistance can build up. Alternate sprays
with Bravo, an effective protectant spray.
Greenguard is an alternative
to Fungus Fighter. It has a protectant/systemic activity and is
a mixture of two different sprays so disease resistance is less
likely to build up.
The chance of plants
becoming infected with fungal diseases can be reduced by providing
optimum growing conditions good air circulation around the
plant, a cool, moist root run, mulching plants in late winter and
mid summer. Feed plants with a specially blended plant food for
acid-loving plants in late winter and again in mid summer.
Another possibility is
a root-rot disorder. If the leaves start to wilt but remain on the
plant, turn yellow and eventually die back, it is possible the plant
has a root-rot disorder. This is caused by poor drainage and the
roots sitting in water-logged soil. There is no remedy. The plant
should be removed and destroyed. Avoid replanting into the same
To avoid root-rot, provide
good drainage plant into raised beds if necessary and incorporate
plenty of peat, compost or organic material into the planting hole.
by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor
of Resource Management.
with permission from NZOOM Home and Garden content,
from the previous
The views expressed here are not necessarily those of the RNZIH