Question
Our flowering plum tree is about 10 years old and last year we noticed galls growing on the roots close to the surface of the ground.  A nursery diagnosed it as root gall caused by nematodes.  We were told no chemicals are available for treatment but there are beneficial nematodes that can be used.  How effective is this type of treatment?  Is it worth trying or would it be best to remove tree which hasn't grown at all this year and start over.  If we were to start over, what needs to be done so a new planting would not succumb to the same fate?

Answer
Hi. To the best of my knowledge adding beneficials probably would be to little to late for this tree. The only thing you might try as a home owner is Soil Solarization for Control of
Soil borne Pest Problems

Solarization is a simple non chemical technique that captures radiant heat energy from the sun. This energy causes physical, chemical, and biological changes in the soil. These changes lead to control or suppression of soil borne plant pathogens such as fungi, bacteria, nematodes, and pests along with weed seed and seedlings.
How to Solarize Soil
The area to be solarized should be level and free of debris and large clods. The soil should be watered if the soil is dry. The soil should not be saturated but have the crumbly damp look that the soil would have at planting. Place clear plastic over the soil surface and bury the edges in a foot deep trench. The plastic can be clear construction grade plastic and vary in thickness from 1 to 6 mils. The thinner plastic
(1-2 mils) will allow better soil heating since it will reflect less solar energy. The
plastic should be left in place for 4-6 weeks. Solarization is most effective when done in June and July. However, depending on your geographic location,solarization may also be effective in May, August, and September. This method only kills nematodes in the 1st foot or so of soil,so after the process is complete than adding beneficials should help down the line when new ones come up from below the solarized soil
Sanitation
Nematodes are usually introduced into new areas with infested soil or plants.Prevent nematodes from entering your garden by using only nematode-free plants purchased from reliable nurseries. To prevent the spread of nematodes, avoid moving plants and soil from infested parts of the garden.Do not allow irrigation water from around infested plants to run off, as this spreads nematodes. Nematodes may be present in soil attached to tools and
equipment used elsewhere, so clean tools thoroughly before using them in your garden.  Hope this helps. You have a tough one!