Fire Blight (bacterium – Erwinia amylovora): The bacterium causing fire blight overwinters at the margins of the cankers formed on twigs and branches in the previous season. Active bacteria are in the healthy tissue next to the canker rather than in the canker. In the spring the bacteria begin to multiply at the same time growth starts. As the bacterium increases, an ooze is formed at the margin of the canker. Insects are attracted to the ooze and it is carried to the open blossoms. Splashing rain can also spread the bacterium. Once bacteria enter a blossom, the blossoms are blighted within 7-10 days after infection. Bacteria continues to be spread further by insects visiting the blighted blossom and carrying the bacteria to adjoining blossoms. Spread by honey bees is increased during periods of warm temperature, sunshine, and still air. After blossom infection, bacteria spread into the fruit peduncle and finally into the twig. Ooze is continually being produced which can add to secondary infection. During periods of high humidity, the bacteria can enter into young leaves. Fire blight is favored by a mean temperature of 60oF or above. The control of fire blight is based on several steps:
- Plant resistant or tolerant varieties: Kieffer, Orient, Garber, or Douglas.
- Maintain balanced fertilizer level. Do not use excess levels of nitrogen.
- Prune during dormant months. Summer pruning may encourage tender succulent growth which is more susceptible to disease.
- Remove overwintering bacteria cankers by pruning. Make pruning cuts 8 to 12 inches below visible sign of disease.
- Apply bactericides on 5 day intervals between early bloom and late blooms.
- Reapply a bactericidal spray to an orchard if it is damaged by hail or receives a heavy rain immediately after an application.
Leaf Blight and Fruit Spot (fungus – Entomosporium maculatum): Leaf spots first appear as small purple spots which enlarge. As the spots get older, they develop purple margins with brown centers. Fruit spots are one-fourth inch in diameter, black, and slightly depressed. They sometimes coalesce to cover a large portion of the fruit surface. Lesions also occur on twigs and are the overwintering inoculum. In the spring, twig lesions produce spores which are washed by rainfall to leaves. After infection there is a period of one week before symptoms are observed. Secondary infection can occur during the spring and summer when the temperature is near 75oF. and surface moisture is on the leaves. Fungicides should be applied at full leaf development and continued at two week intervals for four sprays.
Bitter Rot (fungus – Glomerella cingulata): The fungus attacks apples and pears and enters the fruit through uninjured skin. Infected fruit are characterized by a firm rot which forms a circular light brown spot. With age the spots become almost black and have a saucer-shaped depression. The organism overwinters in decayed fruit and in cracks on the old bark. A broken limb or twig will serve as an overwintering site. Symptoms are first noticeable in mid to late July. A temperature of 85oF and light rain favor development of the rot. Good sanitation will help reduce losses from this disease. Remove all broken limbs and decayed fruit. Once the disease begins to show up, spray at seven day intervals for two or three applications.
Black Rot (fungus – Physalospora obtusa): Black rot is a firm-textured rot. The spot at first is light brown but darkens with age. A circle of raised dark postules are formed in the center of the spot. Infected leaves are covered with many small purple specks. At maturity the spots are purplish cast with brown centers. Twig infections are small, sunken, reddish-brown areas. The organism overwinters in cankers, decayed fruit, and dead wood. In the spring spores are formed in the cankers. Leaves are the first tissue to be infected. A temperature of 80oF. and rainfall encourage disease development. Infection generally takes place at the blossom end of the fruit as it reaches maturity. Sanitation is one of the more important means of control. Remove all dead twigs, limbs, and decayed fruit. Fungicides must be applied when the fruit is beginning to expand in the spring.
Bot Rot (fungus – Botryosphaeria ribis): The fungus attacks both woody tissues and fruit. On limbs, new infection shows up as small blisters. These lesions serve as a source of inoculum for next spring. Fruit infection results in small, reddish-brown spots which develop rapidly causing a soft rot. A temperature of 75oF or above favors the development of the fungus. Bot rot is most damaging on weak trees. During periods of rapid twig growth, the diseased area will be sloughed off. Prune out any dead or diseased wood. Fungicides will need to be applied from immediately after bloom until near harvest.
Pear Scab (fungus – Venturia pyrina): Symptoms of apple scab and pear scab are similar except pear scab will attack twigs. Twigs are a source of early spring inoculum. The control program for apple scab will also control pear scab.
Stony Pit (virus): This disease is one of the more common viruses found on pears in Texas. The outer flesh of infected fruit is gnarled and deformed. When the fruit is cut, it has brown, hard structures scattered throughout the flesh.
Bitter Rot, Fly speck, Sooty Blotch: (See section on Apple)
Cotton Root Rot: (See section on Cotton Root Rot)
Mushroom Root Rot: (See section on Mushroom Root Rot)
Crown Gall: (See section on Crown Gall)
|Pear Varieties and Their Reaction to Fire Blight|