What indicates a sudden unexpected disease outbreak in crops?

A rapid increase in disease severity is a clear indicator of a sudden unexpected disease outbreak in crops. This increase suggests that a pathogen is spreading quickly and affecting a growing number of plants, which can significantly impact crop yield and quality. Observing such a spike in disease severity may prompt further investigation into the underlying causes, such as environmental stresses, pest infestations, or the presence of new disease pathogens.

The other options do not suggest indicators of disease outbreaks. Consistent crop growth typically signifies that plants are healthy and not under stress, while low pest populations would generally correlate with reduced disease risk. Stable environmental conditions can also mean that there are no significant changes to create an outbreak, as many diseases thrive in fluctuating environments. Thus, they would not be associated with sudden disease occurrences in crops.