What should a marketing manager do when he or she has incompetent area sales managers?

Why is the Sales Manager incompetent, is he facing any issues? Is the M strategies giving a hard time to S functions? As a marketing manager, i'd talk to the Sales team and find out what their issues are and try to go with a different approach incorporating their suggestions, if all fails, then yes, replacement, but i don't think that will be a solution either unless you have a good rapport with the sales manager and equip the person to face the market proper.

It is to be said in other words that maybe something goes wrong with sales department or at least marketing and sales departments are not working cohesively.
indicating to incompetency of sales managers is not clear enough to solve the problem.
review the sales and marketing processes particularly interlinks between sales and marketing departments to precisely find the real causes.

If an Area Sales Manager (ASM) is incompetent, sales revenues will take a hit.
Marketing Manager needs to immediately evaluate the situation and come up with a plan on how to recover the sales figures.
An apt solution will depend on considering various factors applicable for this company but the plan may call for replacing the incompetent ASM and either promoting a deserving candidate from within or hire a new ASM.

i Believe in Human resources, so I would go for training and I should listen to the market through the Area sales Manager point of view, and find the strength points and do motivation .
and the weakness points work on it through the follow up and daily reports.

Incompetent is a very strong term.
In present recession and highly competitive environment sometimes a very well established company is not able to perform.
In my opinion it is responsibility of the management to help improve competence levels of their managers through various techniques and strategies.
However one step could be to organize a training program to strengthen their weak areas.
Secondly sometimes at certain location a manager somehow is not able to perform he / she can be transferred to another location, my observation is that sometimes poor performers start performing well at new locations.
Every company has its own strengths and weakness, in my opinion every company carefully employs after due diligence process of selection therefore after their induction in the company it is management's challenge to provide sufficient resources so that managers apply their full potential and skills to achieve company objectives for which they have been selected by the management.

Incompetency will arise only he or she doen't get sufficient knowledge about the company or product or his academic background.
First we will sent that Area sales manager for through training about the products and slowly increase his confidence level among the company and products.
Marketing manager also motivated by announcing small competition and rewarded him/her according to their sales records.

he or she must be covered these area as add duty to all representative until he filling these vacant area