White mould on bay leaf plant

(Question)

I have an indoor bay leaf plant, which has what I believe is a white mould. The plant has lost most of its leaves.  The leaves progressively turn a lighter colour on both sides as they die off.  You can see the progression as the disease works it way across the individual leaves and then across the plant, from the edge of the leaf to the stem. As the leaves turn colour, they become dry, stiff and dead, and are smooth, not lumpy. The discolouration could simply be a result of the leaves dying, but a line is visible on the leaf between the live and dead portions.

I have not changed the way I maintain the plant. I water once weekly and I keep it in the same spot, by the window. The plant soil is generally drier. It is not close to a door and does not get a draft of cold air that could damage it. I have added some plant fertilizer in the water but not recently as it seemed very healthy.

I had this problem once before about 2 or 3 years ago. At that time I removed all the infected leaves, washed off the remaining leaves and it came back. Now I have done the same sort of thing. I washed the plant with alcohol. Then I sprayed it with Sunlight dish soap and then rinsed it with water. The remaining few leaves seem to be okay now (at least, it seems to have stopped moving across the remaining leave). The plant only has about 6 leaves left, having lost well over 100 leaves. One of the remaining leaves has a bit of disease on the edge but it seems not to have progressed across the leaf. It only took about a week for the plant to die off. By the time I noticed the problem, most of the plant had been affected.  There is no sign of bugs, and the plant is isolated from my other plants.

Any suggestions for me?

 

 

(Answer)

Bay leaf plant (Laurus nobilis) is one of my favourite houseplants.

From your description, your plant likely has powdery mildew.   Powdery mildew looks like a whitish to greyish powdery coating on the leaves, and it may affect leave, buds and stems (i.e., the above-ground plant parts). It is caused by fungi and is common on indoor plants during the winter (it’s now early March), when conditions like low light, poorer air circulation and cooler temperatures may stress your plant. Fungal spores are spread by air currents or splashes of water.  Oddly, in houseplants, powdery mildew may be most severe when the conditions are relatively dry.

The following points are important (and you have done many of these things already):

  • keep the plant in a well-ventilated area (it needs some air circulation)
  • don’t overcrowd the plant (this decreases relative humidity levels) [yours is on its own, so this is not a concern]
  • don’t mist the plant
  • make sure you do not wet the leaves when you water the plant
  • isolate the plant (you have done this)
  • remove the infected/infested plant material (dispose of  in in a plastic bag and seal it – put this in the garbage, not compost).

Bay likes a sunny, warm spot – and yours has obviously been happy for a number of years, right where it is.  You mention that the plant soil is generally kept dry – to decrease stress on the plant, you might want to water it a bit more frequently than once a week – maybe every 5 days or so, don’t let the soil get too dry.  However, if you over-water the plant, this could increase the likelihood the fungus returns.

The cultural practices described above are preferred treatments for dealing with powdery mildew.  However, fungicides may be effective in controlling powdery mildew.  If you want to use a fungicide, first check with your nursery, as many cannot be sprayed inside the house (and make sure that your bay plant can tolerate the chemical).

In severe cases of powdery mildew, the plant may have to be discarded.

Sometimes, scale can be confused with powdery mildew.   Scale insects may excrete a honeydew that accumulates on leaf surfaces, and this can be colonized by a fungus called sooty mould.  This would be a dark colour (it looks like soot) as opposed to white, as in your plant’s case – so I don’t think that your plant is afflicted with scale.  However, if this is scale, removing all the diseased plant material, and using a Q-tip dipped in rubbing alcohol to the affected areas (as you have done), should do the trick.

Here are a couple of  good articles:

1) Clemson Cooperative extension, Houseplant diseases and disorders.

2) Cornell University Cooperative Extension Rockland County. Houseplant pest management .

All the best with saving your lovely bay plant!