I bought this Schefflera three weeks ago. I changed its pot from the plastic tub it came in to a ceramic one. Last week, one branch died completely; all the leaves withered. I thought I might be overwatering, so I cut back. Now the other two branches have begun to have leaf issues similar to the dead one (the white spots are dust from the store where I bought it).
Should I continue to not water this guy, or should I look for some other cause?
The white spots are not powdery mildew. They are a mixture of pesticide residue and hard water stains from overhead watering in the grower's greenhouse. Powdery mildew is usually "fuzzy" and residue is flat on the leaf.
Either way wiping the leaves with a cloth is sufficient as a first stage. If it is powdery mildew it will return, residue will not.
The necrotic (dead) spots on the leaves are typical of root rot but can also happen if the plant has other types of fungus/virus/bacterial infection.
Given that you have repotted it the best solution is:
If the plant continues to deteriorate you should return it to the store as it may have been sold to you with a fungus/virus/bacteria. A healthy plant under good conditions can outgrow minor issues.
Just a note on the "dust". Dust will easily wash away with water. The appearance and damage to the leaves and stems makes me think it might be powdery mildew, a fungal disease that appears as a white powdery coating on the leaves.
My experience and reading on powdery mildew is that much of it can be just washed off the plant if caught early. Once the spores attach themselves to the leaves you need something else that will help. I've found that neem oil is effective against powdery mildew and posted some more information on my site.
Another option is a diluted milk spray (1 part milk 9 parts water). I've used this as well and it does a pretty good job against powdery mildew but seems to be best to use on a regular basis before the you see signs of the disease. It's been something I've used and looked into over the years and have published some more info regarding what makes milk a good fungicide on my blog.
Lots of people have seen positive effects of using diluted milk as a foliar spray to control disease. See Tomatoes Like Milk. In recent years there have been a number of studies regarding milk and plant pathogens to figure out how milk helps. It was first thought to be that the calcium strengthened the immune system in plants but now it looks like it's the probiotics in milk that are the key. Various bacteria that have been identified in milk are effective against a wide range of plant pathogens including powdery mildew.
