1. Powdery mildew and smut (distinctive white-buff angular spots on the leaves) are the most common fungal diseases. Smut tends to occur early in the season in Auckland when conditions are damp and cool, white powdery mildew usually shows up after Christmas. In both cases apply a fungicide early in the season to prevent a build up of infection.
2. Aphids and Earwigs can be a problem early in the season and caterpillars become evident at show time (Jan-Feb). All can be controlled by insecticides.
3. Mites can be a problem in dry seasons. Look for brownish curled leaves that have tiny cobwebs on their under-surfaces. Apply a miticide. Insecticides can make things worse by killing off insects that prey on mites. Apply the miticide along with your fungicide sprays early in the season. By the time mite symptoms are easily seen the populations are usually high and difficult to control.
4. Sclerotinia attacks stems. Soggy areas and black bodies (which look like rat dropppings) appear inside the hollow stems. The disease thrives in wet seasons. Hygiene is most important and old flowers should be removed to prevent their petals dropping onto stems and leaves - this is called deadheading. Never put dahlia compost/mulch back onto dahlia beds.
5. Viral diseases are the biggest problem as they cannot be cured. Plants with stunted or twisted growth are in many cases virused, and infected plants should be destroyed to stop viruses spreading.
Tomoto spotted wilt virus (TSWV) and cucumber mosaic virus (CMV) are the two most common viral diseases in New Zealand. Symptoms of the TSWV are a wilted appearance and blotchy leaves. Plants infected with CMV show lighter areas associated with the leaf veins. In both cases the shoots may have a crinkly or curly appearance.
Recipes for Earwig Bait - Auckland Dahlia Society
Wheat Bran or Oatmeal 225g / 8oz
Maldison emulsion 30ml / 2 Tblspn
Blood & Bone or meat meal 100g / 3.5 oz
Sugar 100g / 3.5 oz
454g / 1lb of the mixture should be enough for 100 sq yards
Wheat Bran or Oatmeal 454g / 1lb
Molasses or treacle 150ml / 0.25 pint
Malathion powder 30g / 1oz
Water 454ml / 0.75 pint
Dissolve the malathion powder and molasses in water and add to the bran. A friable mash should result. Bait should be sprinkled around the soil surface in the late afternoon - preferably in late spring. Several applications may be necessary.
Scrunch up a piece of dry newspaper and place on the top of your Dahlia stake. Cover with an empty baked bean (or other) tin.
Check every 2 or 3 days and open the paper over a bucket of hot water to dispose of earwigs.