Question
QUESTION: What shall I do to stop the browning of the leaves of my newly purchase sago palm plant. I didn't transfer it from its container but place in full sunlight in my front yard,

ANSWER: gene:
Difficult to say exactly what the cause here may be, but a couple of possibilities may be too much sun too soon.  If the plant was previously in considerably less light, then full sun may be a sudden shock. Always a good idea to gradually "aclimate" the plant to prevent this from happening.  Another possibility may be related to soil conditions in the pot.  Eventhough these plants prefer a dryer/well drained soil, the plant may have dried down too fast and maybe too quick.  Her again a gradual transition is better.

I do not suspect an infectious type of issue, but rather an environmental one based on your description.  Hopefully, your plant will have an opportunity to adjust before too much tissue is affected and not able to recover from the shock.

Steve

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QUESTION: So what shall I do? Do I have to transfer the plant to a shaded area or just keep on watering. I place this in my front yard cause it is the main focal point. Can you suggest a plant that is similar in looks that could tolerate full sun as well as our weather here in Washington State. How about Agave or a yucca plant

Answer
Gene:

If the plant has been in full sun for several days, we may just wait to see how it fares.  There is a temptation to put the water to it, but be careful not to overwater or else this may compound the issue.  The soil needs to be well drained.  The pot should have at least one drain hole to allow water to flow out the bottom.  Since the leaves have been affected, they are not going to help pick up water as much as if they were all green. This is a slow growing plant, so you may have to wait a while (3-4 weeks)and watch for any new leaves forming at the base of the plant. A bit of TLC may be called for.

The two other plants that you mentioned grow well in full sun and do well in warm/tropical climates.  They will need to be sheltered during the winter.

Steve