Petal Blight in Rhododendron spp.

The buds and flower is the only part affected by this fungal disease. Humid conditions favour the spread of this fungus, which initially starts as small circular spots that quickly enlarge into irregular blotches.

Affected flowers dry out and turn brown and papery. They remain stuck to the plant for some weeks, unlike healthy flowers which would fall from the bush once faded. Grey mould can look similar and only affects petals.

The specific fungus particularly affects Kurume and Indica azaleas.

Control by collecting and destroying dead and diseased flowers, but spores can blow from plants considerable distance away. Spray with triadimefon as soon as buds show colour in humid weather if severe infection.

Reference : Judy McMaugh, pg 269

Iron Deficiency in Griselinia lucida

This example of iron deficiency was photographed in the RTBG.

When a plant is unable to take up enough iron, leaves become light green fading to pale yellow or even white. This happens gradually and the veins remain green, until the end of the process. Youngest leaves are affected first. In mild cases, the leaf size and new growth appear normal, but if severe, can reduce the size of the leaf and cause dieback. Fruit crop will be reduced.

Handreck suggests a quick and easy way of checking whether yellowing of leaves is due to iron deficiency : spray the plant with an iron chelate. Greening within a week indicates iron deficiency.

Problem often occurs as a result of soil pH being too high. An attempt to lower the pH can be made but this is a slow means of correcting the problem.

Handreck recommends treating the deficiency with the use of an "iron compost" which essentially involves adding iron sulphate to either compost, peatmoss or animal manure in the ratio of one cup (about 230g) to 10L, then digging holes around the rootzone and filling holes with this mix (see Gardening Down Under pg. 146).

Reference : Judy McMaugh pg. 254, Gardening Down Under, Handrek