Many growers rely on good agricultural extension services to successfully grow their crops. Correct field diagnoses of plant health problems are an essential starting point. However, some agricultural extension workers may have had limited capacity building in field diagnosis during their agricultural education, and many may have had limited or no continuing education possibilities during their advisory work life. As a result, errors may occur in the course of diagnosis of plant health problems with devastating consequences. The objective of this study, therefore, was to assess the value of short and inexpensive in-service training that is intended to improve the quality of field diagnosis of plant health problems. Eight groups of agricultural extension workers totaling 165 participants were trained during a 3-day-long intensive course in the diagnosis of plant health problems as part of the plant doctor training in the Plantwise program in Burundi (44 ± 9 years of age, 14 ± 10 years of experience in agriculture and 8 ± 7 years in extension; 10% females). Empirical data from before-versus-end and after-course analyses revealed that such courses can indeed increase knowledge and skills in field diagnosis by 20 ± 8% points during the training and by 9 ± 4% points when having worked for 8 to 19 months after training. The level of variability among extension workers was also reduced by 0.8 ± 1% points pre-course versus end-course, and by 1.1 ± 0.3% points from pre- versus post-course. Interestingly, there seemed little to no influence of work experience in agricultural extension on the quality of field diagnosis of plant health problems. More advanced educational levels only slightly improved the diagnosis quality of extension workers; gender played no role; younger workers performed slightly better than older ones. This may indicate a lack of opportunities for further training as part of continuing adult education. Our results suggest regularly repeated in-service training for agricultural practitioners may be beneficial. Investments in such training seem worthwhile since better diagnoses can lead to more appropriate pest management advice and judicious use of pesticides.