Licensing laws vary from state to state. Check with your own state's agricultural commission OR your local extension office for the information you need. It is essential that you follow both the Federal laws AND your state's laws. The penalties can be substantial. Some state requirements are strict while others pretty darned lax.

The ability to diagnose plant problems accurately comes with lots of experience, training, and education. One learns to look at and evaluate site condition for abiotic causes as well as studying up on the usual insect, disease, or other biotic culprits.

If you don't have the knowledge of how to diagnose plant symptoms, then you need to get some education. Many community colleges have horticulture classes, and they are often filled with adult students just like you who want and need to improve their job skills.

It's good that you doubt yourself. There's nothing more 'dangerous' than a gardener with pruners in one hand and a sprayer filled with chemicals in the other, but who doesn't know an azalea from a holly, or a ladybug from an aphid. And you are SO right in recognizing that a particular plant symptom such as leaf browning can be caused by several different factors. It takes a good diagnostician to figure out the problem with certainty.

Good luck to you.

1st I am a fairly smart man. Poor enough to not afford books in collage and still getting a 3.8 grade. Farmed since big enough to push the clutch, and a 15 year master mechanic on marine engines. I have a pestiside lience (no I never could spell)I have talked a person thru getting a boat running 20 miles off shore 2000 miles away. When it comes to plants I think you need training close to dr. status . Insurance might be less but the range of problems are pretty close. The more I learn the less I know I know. Tell them it is your best guess but remind them it is only a guess, put it in writing and have them sign it. Wife is a lawyer and made me include the last line. LOL

I agree with rhizo - licensing requirements differ from state to state but in most cases, no license is needed to apply an OTC general use pesticide (like RoundUp) at the owner's/client's request. And there are never any guarantees :-))

As to being able to diagnose plant problems, that is often a case of many years of experience combined with proper training. Many local extension offices can diagnose plant issues and recommend treatment or they have access/referrals to plant pathologists that can make the ID accurately. And the extension offices provide these services for free (although plants sent to a pathologist for diagnosis generally will have a fee associated with them). Some better retail garden centers will also have certified horticulturists on staff who have sufficient training and experience to diagnose many plant problems and offer recommended treatment. I get as many professional gardeners/landscapers in with plant questions and disease/pest issues as I do home gardeners!! No one is expected to know everything :-)