According the University of Maine Extension web site, 'Pruning is a vital part of maintaining a healthy raspberry planting. This practice greatly inhibits the spread of raspberry diseases and improves fruit quality and yield. During the summer months, regularly remove all new canes that emerge outside the desired plant row width of 12 to 18 inches. This improves light penetration and air circulation for the canes in the middle row that will fruit next year. Also r...