Analysis of Fire Blight Shoot Infection Epidemics on Apple

Fire blight of apple (Malus × domestica Borkh.) can be found throughout the United States and many other countries and is a serious problem on trees of susceptible apple cultivars (9). Fire blight is caused by the bacterium Erwinia amylovora and occurs also on pear, quince, Rubus spp., and several other rosaceous hosts. The initial inoculum in an established orchard comes from overwintering cankers that remain from prior growing season infections. Bacteria that exude onto canker surfaces are spread by insects to blossoms where infection can occur under appropriate environmental conditions. Secondary inoculum produced from infected blossoms can cause repeated blossom infections, as well as infections of tender shoots later in the growing season. Primary inoculum for the infection of shoots also can come from shoots that emerge from buds adjacent to overwintering cankers. Epidemics of the shoot blight phase of fire blight occur sporadically and vary in intensity from year to year depending on the amount of overwintering inoculum, host susceptibility, crop management system, and environmental conditions (23). In the extreme, shoot blight epidemics can be economically devastating on a regional level. Because of the perennial nature of fruit crops, losses occur as a result of direct crop loss in the year in which disease occurs, as well as past and future development costs related to raising trees to the stage of productivity. In recently established apple orchards, total orchard loss can occur under a sequence of favorable conditions that lead to low to moderate levels of primary infection followed by a traumatic event or events (e.g., frost, hail, high wind, and rain) on a regional level that promote widespread inoculum dispersal and greatly increased host susceptibility due to wounding. The 2000 epidemic in Michigan is a good example in which the southwestern region of the state experienced unusually warm, humid, and wet weather in May (favorable for blossom infection), which was followed by frequent heavy rains and hail during the period of succulent shoot growth. Losses from the 2000 epidemic in Michigan were estimated to be near $42 million (Mark Longstroth, personal communication). Maryblyt is a computerized model for the prediction of fire blight that is widely used by producers, extension advisors, and researchers (16). The program has various modules for the prediction of blossom and shoot blight. Several research articles have been published that provide data to validate the blossom blight component of Maryblyt (4,6,8,22). No one has attempted to validate the shoot blight portion of the model. Prediction of first early shoot blight symptoms in Maryblyt is based on the accumulation of degree days following the first appearance of a source of secondary inoculum, either blossom blight or canker blight, whichever occurs first; however, because wind and insects can disperse the pathogen from other orchards and wild trees, the primary inoculum sources need not necessarily occur within the orchard being monitored. As with blossom blight, dispersal of the pathogen to shoots and foliar surfaces prior to or during an infection event are highly probable (19). Infection of epiphytically or endophytically colonized shoots has been attributed to wounds caused by insects with sucking or piercing mouthparts, and bacterial ooze spread by rain, insects, aerosols, and birds (19). Steiner observed that, following the appearance of first shoot blight symptoms, later shoot infections occur more or less at random (16). However, the pattern of shoot blight development in an orchard has never been studied. An epidemiological study of the progression of several natural shoot blight epidemics was conducted in replicated blocks of four apple cultivars. The objectives of the study were to validate the shoot blight component of Maryblyt and describe disease progress over the growing season and to determine the temporal and spatial dynamics of apple shoot infection. Such basic knowledge of disease development could provide more specific recommendations for use of bactericides, insecticides, biological controls, or removal of ABSTRACT Biggs, A. R., Turechek, W. W., and Gottwald, T. R. 2008. Analysis of fire blight shoot infection epidemics on apple. Plant Dis. 92:1349-1356.