You are examining a field of crop plants and notice that many of the leaves have small scattered necrotic spots on them, but none are completely diseased. You then examine the shoot apical meristems of the plants and notice that the most recently formed internodes and leaves are expanding to their usual size. You conclude that the infection is likely:

Avirulent.

Explanation:

VIRULENCE is the ability of a pathogenic organism  to infects the host, leading to damages  or death of the host. The extent of these virulent effect depends on  certain chemical substances ( called Virulence factors) produced during the  pathogenic processes.

The virulence effects  is achieved  due to the ability of the virulent factor to disrupt the entire physiological mechanisms of  the organisms; e,g crop plants; though  suppression of the host immune response, disruptions of the immune mechanisms, colonization of the host DNA structure etc. Therefore the pathogenic effects suppressed the host resistance and spread throughout the host body system.

In this present scenario, the pathogenic effect of the likable bacteria; is not virulent, because

none of the d crop pant is completely diseased.

the nascent intenodes and leaves  are growing to usual size.

Consequently, the physiological and the morphological features of the crop plants are still intact. Thus the infection is AVIRULENT.