Final answer:
Letter reports are desired when there are significant weather events or issues affecting the crops.
Step-by-step explanation:
In the case study provided, it does not explicitly mention a period report or letter report. However, based on the information provided, it seems that the letter reports are desired when there are significant weather events or issues affecting the crops. The passage mentions that in cases of beneficial rain, fully grown crops, drought, violent weather, floods, or crop damage, the acreage affected should be reported in writing by the end of the eighth month. For the correctional issue paper assignment, the interview write-ups are due before the final paper, implying that preliminary reports such as letterreports should be prepared well in advance to meet the final submission deadline.