Students in Plant Pathology 501, "Diseases of Ornamentals" on the OSU Main Campus spend a good amount of their class time outside, checking out plants and infectious and non-infectious diseases in the Chadwick Arboretum and its environs. Last week, the class talked of bacterial fireblight of pears and the fact that years in which it is especially bad are years in which infections really get started with warm (60F and above), wet weather during bloom. Of course, in many areas of Ohio this year, including Columbus, pears were in full bloom during the mid-March period of 70s and 80s in which heat units were accumulating rapidly. So, these were seemingly ideal conditions for bloom infections, presaging a big year for fireblight incidence.
With that reality, as the class walked on one field trip on campus last Thursday, noting a line of pears with clean foliage, the class discussed why the class did not see any symptoms of fireblight. No "shepherd's crook" of curled shoots with blackened leaf petioles. No greasy brown to black foliage hanging on to the twigs. Did we get the "environment conducive to disease" aspect of the Disease Triangle wrong? Were these all fireblight-resistant cultivars of Callery pears? Not so, since we had seen fireblight on these pears in previous years. So, why no symptoms of fireblight on these susceptible plants, with such conducive environmental conditions, with the presumed presence of the pathogen from overwintering stem cankers?
The answer came in class this Tuesday, only five days after observing clean foliage last Thursday. The answer came in the form of hundreds of fireblight strikes on the trees noted this Tuesday. This is a reminder of the latent nature of disease infections. The bacterium entered the blooms and leaves of the plants through wounds earlier, as splashing water and pollinating bees spread the bacteria, but the results of these infections, the damage to leaf and stem tissue from the growth of the bacteria, did not result in symptoms until now. Because infections by microscopic pathogenic inoculum is invisible to the naked eye and symptoms lag behind infections for days and weeks, it is often a surprise for observers to see what seemed like completely healthy tissue suddenly show symptoms of disease from infections that occurred much earlier.