Optimum temperature for growth is 27☌ (81☏), with cell division occurring at temperatures ranging from 5 to 31☌ (41 to 88☏). It grows on most standard microbiological media and on several differential media. amylovora is classified as a facultative anaerobe. amylovora are gram-negative, rod-shaped, measure 0.5-1.0 x 3.0 mm, and flagellated on all sides (peritrichous) ( Figure 9). M.26 and M.9 rootstocks are highly susceptible to the pathogen.Įrwinia amylovora is a member of the family Enterobacteriacae. Symptoms of rootstock blight can be confused with Phytophthora collar rot. Red-brown to black streaking may be apparent in wood just under the bark ( Figure 8). The bark of infected rootstocks may show water-soaking, a purplish to black discoloration, cracking, and signs of bacterial ooze. Rootstock infections usually develop near the graft union as a result of internal movement of the pathogen through the tree or from infections through water sprouts or burr knots. Severely diseased fruits blacken completely and shrivel. Droplets of bacterial ooze may form on lesions, usually in association with lenticels ( Figure 7). Indeterminate, water-soaked lesions form on surfaces of immature fruit and later turn brown to black. Wood under the bark will show streaked, brown to black discolorations. Amber-colored bacterial ooze mixed with plant sap may be present on bark. At advanced stages, cracks will develop in the bark, and the surface will be sunken slightly ( Figure 6). Bark on younger branches becomes darkened and water-soaked ( Figure 5). Infections initiated in blossoms and shoots can continue to expand both up and down larger branches and limbs. Numerous diseased shoots give a tree a burnt, blighted appearance, hence the disease name ( Figure 4). Leaves on diseased shoots often show blackening along the midrib and veins before becoming fully necrotic. Tips of shoots may wilt rapidly to form a "shepherd's crook" ( Figures 1 and 3). Shoot symptoms are similar to those in blossoms but develop faster. Ooze droplets are initially creamy white, becoming amber-tinted as they age. During periods of high humidity, small droplets of bacterial ooze form on water-soaked and discolored tissues ( see example on fruit, Figure 7). Similar symptoms often develop in the base of the blossom cluster and young fruitlets as the infection spreads internally ( Figure 2). Later these tissues shrivel and turn brown to black. The floral receptacle, ovary, and peduncles become water soaked and dull, grayish green in appearance. Blossom clusters and young shootsīlossom symptoms are first observed 1-2 weeks after petal fall. Generally, symptoms of fire blight are easy to recognize and distinguishable from other diseases. Symptoms of fire blight can be observed on all above ground tissues including blossoms, fruits, shoots, branches and limbs, and in the rootstock near the graft union on the lower trunk. Today, fire blight is an important disease of apples and pears in many parts of the world. It was the first bacterium proven to be a pathogen of plants. HOSTS: Apple, pear, several rosaceous ornamentalsĮrwinia amylovora is a native pathogen of wild, rosaceous hosts in eastern North America.
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