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Plant Pathology


Fire Blight

Although not a serious problem every year, fire blight is a disease which can "explode" with devastating consequences under the proper conditions. Consequently, fire blight should be monitored closely.

SYMPTOMS: Infection first occurs during bloom. Infected blossoms appear water-soaked and turn brown or black. The infections may then spread through the blossom-bearing stem (pedicel), turning it black, and into the spur or main branch to form a canker. As young shoots begin to develop, they may also become infected. This "twig blight" phase of the disease begins at the succulent growing tip and moves downward. Infected twigs turn dark brown and become hook-shaped, like the top of a cane. The leaves on infected twigs or spurs then turn brown and die and appear as though they have been scorched by a fire, but remain attached.

Fire Blight

If the fire blight bacteria spread into the main branches, they may cause a canker to develop. Such cankers often appear outlined by cracks in the bark at their margins. Fruit may occasionally become infected; these generally turn brown to black at the site of infections, and exude droplets of milky or amber ooze. In fact, the presence of ooze on the surface of any diseased tissue, especially during humid weather, is a good diagnostic sign for fire blight.

DISEASE CYCLE: The fire blight bacteria, which overwinter in the margins of limb and trunk cankers formed the previous season, begin to multiply rapidly as temperatures warm in the spring to about 60 F or higher. As the bacterial population increases, oozing sticky droplets will be formed on the surfaces of cankers. The bacterial from these droplets are then spread to open blossoms or other plant parts by insects and splashing or wind-blown rain where they may continue to multiply. When bacterial numbers become high on the flower surface, they are capable of causing infections, but only after they have been washed by rain or heavy dew to the nectaries (natural openings at the base of the flower). Further spread from diseased flowers to additional blossoms or shoots is accomplished by insects and splashing or wind-blown rain.

Thus, the 4 factors needed for primary infections in the orchard are a) open flowers, b) high numbers of bacteria, c) a little rain or dew to move the bacteria to the nectaries, and d) a warm temperature at the time of infection. A grower who monitors these 4 factors can learn to anticipate infections and also to prevent them with the aid of a computer program called Maryblyt. A daily record of the flowering status of the orchard, the maximum and minimum temperatures, and a rain gauge are all the information that is needed to run the Maryblyt program. Growers who do not have their own computer sometimes can get help from their county extension office.

After primary infections, bacteria build up to very high numbers in the dying blossoms and fruitlets. From these sources, the bacteria launch an attack on the rapidly growing shoots, causing shoot blight and death of terminals in the tree. Infection from these blighted terminals may spread into branches and limbs where they become a source of overwintering bacteria for next year's disease outbreak.

SCOUTING: Be especially alert for the blossom blight and spur blight phase of this disease when the weather is warm (65-800 F), humid, and rainy during the bloom period; similarly, be especially alert for the twig blight phase if these conditions prevail while active shoot growth is occurring. All apple varieties are at least partially susceptible to fire blight; however, the following varieties are highly susceptible and should be watched closely: Beacon, Fuji, Gala, Idared, Jonathan, Lodi, Paulared, Rome, Tydemans Red, Wealthy, Yellow Transparent and York.

During the bloom period, examine 20 blossom clusters on each of 5 limbs per tree and record the number of fire blight strikes. After the bloom period, similarly note the number of strikes per 100 spurs and 100 terminal shoots on each tree examined. Continue monitoring until the terminal shoot buds have set.

When scouting for fire blight, be sure to check for the presence of blighted suckers arising from the rootstock. Infection of the root-stock can quickly lead to the death of the tree.

Growers with a maximum/minimum thermometer and a rain gauge in the orchard, and records of tree development can use these daily records to manage fire blight disease. With the Maryblyt computer program, it is possible to know how close the orchard is to becoming infected. When risks of infection are high, appropriate sprays can be applied; when risks are low, no sprays will be needed. In the event that an infection has occurred, the computer program also tells the grower when first symptoms should appear so that the grower can break out infected flower spurs before the disease gets out of hand.

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