Q: Are king sagos (cycas revoluta) subject to Ganodema butt rot?
Q: Are king sagos (cycas revoluta) subject to Ganodema butt rot? If not, is there some other disease that has similar conks that cycads would be subject to?
A: After looking at the photo you sent, it is quite probable the causal agent was Ganoderma. Ganoderma, Ganoderma zonatum, attacks palms and hardwood trees so there is no reason to suspect cycads (sagos) would be an exception. There are numerous types of shelf fungi but the quickness of the destruction of the plant points to ganoderma. The only way to know for certain is to examine the trunk tissue after the sago is cut down. The tissue will show a dark ring inside the trunk.
This fungus decomposes the lower part of the trunk palms or trees. It is also possible for the fungi to be present without seeing the conks. Once the conks appear on the trunk, the tree is quick to decline. There are no chemical controls for the disease. The palm should be removed and destroyed as soon as possible. Remove all of the stump tissue along with the soil and do not plant another palm on the same site as these fungi live in the soil.
Keep the palms and cycads healthy by avoiding over irrigation and fertilization. Palm fertilizers should be applied every spring, summer and fall using a 4-1-6-2 ratio (N-P-K-Mg respectively). Fertilizer should be broadcast under the canopy of the palm or cycad. Fertilizer should not be found in large clumps under the palm or allowed to stay on the fronds of the palms. It should never be sprinkled around the bud area as fertilizer only benefits the plant if is can be absorbed by the root tissue. Each fertilizer bag will provide instructions on how it should be applied properly, please read and follow the directions on the label.