I am growing pumpkins for my Granddaughter and the leaves on one plant have a greyish dusty looking deposit on them. I have tried spraying with soapy water but as soon as they dry it returns. Will it kill the plant or damage the pumpkin? Any suggestions please as to how I get rid of it!
The soapy water only works for aphids or other bugs... what you have sounds like a fungal infection... most likely Downy Mildew or Powdery Mildew, common in pumpkins and other gourd plants this time of year. Symptoms of Downy Mildew are very similar to Powdery Mildew. Downy mildew will cause the upper sides of leaves to yellow and brown out and produce grayish/white fungal masses only on the underside of leaves. Powdery Mildew will produce white fungal masses on the upper and lower leaf surface and the stem ruining quality if left uncontrolled. A fungicide from your garden store will kill it, but will need several applications. Try anything with Daconil... such as Ortho Multi-Purpose Fungicide (Daconil 2787) if it's available in the UK. I've used it successfully for several years here in the U.S. Best of luck!
I agree with Clanad, it definately sounds like you have powdery mildew, the white powdery coating is actually the fungal spores and its best treated when symptoms first appear. You can also spray or dust with a fungicide ie carbendazim or dust with sulphur and as mentioned,repeat as necesary.
A preventative measure is toreduce the posibility of the infection by making sure that plenty of water is given around the roots to prevent the soil from drying out completely. Good Luck Tbird+