Question
When I BOUGHT THE ABOVE PLANT, IT APPEARED TO BE DUSTY AND I
WASHED THE LEAVES WITH WATER BUT ABOUT 2 WEEKS LATER SOME OF THE
LEAVES HAD SPOTS  AND SOME WERE DYING, AND I FOUND SOME WHITE
POWDERY SPOTS MAINLY ON THE UNDERSIDE OF THE LEAVES SO WASHED THE
LEAVES AND STEMS AGAIN WITH A DETERGENT BUT IT'S COME BACK AND
MORE LEAVES HAVE DIED.  I HAVE NOT ENCOUNTERED THIS TYPE OF BUG OR
FUNGUS BEFORE.  ANY IDEA OF WHAT IT IS AND HOW TO TREAT IT?  THANK
YOU.

Answer
Dear Marlene,

Not an easy plant to grow but how spectacular it is, isn't it?

You will find that this plant, whilst often appearing to have died, is really dormant and just as you think it is the end of the road it will suddenly throw out new leaves.  

Have you managed to determine if there are insects on the plant or it is mildew?

Red Spider Mites just love a good feed on Alocasia Polly plants - they will suck the sap from leaves and stalks and leave a deposit of white webbing between the leaves and stems.  Mealy bug which are small bugs are covered in a white cottony fluff and you will find these on the stems and under the leaves.  

If your plant does have insects and they keep recurring even though you are washing the leaves and stems then you need to buy a proprietary insecticide and use as per the instructions.

If it is mildew then you will need a fungicide which should be sprayed on the plant.  If the leaves are badly affected then cut them off/

Alocasias like high levels of humidity and to be positioned in bright light and indirect sun.  Keep the compost moist and make sure it is not in a draught.  Clean the leaves frequently.

I do hope you manage to save your plant but if the leaves do all die off then just keep the plant in a shady place, water occasionally and wait for new growth to appear.

Good Luck

Diana