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 60 degrees or above. The control of fire blight is based on several steps:
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 75 degrees F. 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 85 degrees F 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 80 degrees F. 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 75 degrees F 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.)
Keiffer R
Orient R
Garber MR
Douglas MR
Bartlett S
S = Susceptible
MR = Moderately Resistant
R = Resistant
Bacterial Canker (bacterium - Pseudomonas syringae): Cankers develop at the base of infected buds on trunk and scaffold limbs. Cankers spread more rapidly above the point of infection than below and only slightly to the sides. This results in a long, narrow canker. Cankers develop during the fall and winter but are not visible until late winter and early spring. Damaged areas are slightly sunken and somewhat darker in color than surrounding bark. As the trees break dormancy in the spring, gum is formed and flows down the outside of the tree. Cankers have a soured smell. Cankers start development in the fall at the base of a bud. The infected areas increase in size during the cool fall months. If infection takes place early in the fall when the trees are still active, the area is walled off by callous and no further development occurs. The bacterium is a weak pathogen and causes serious damage only when a tree is in a near dormant condition or weakened due to unfavorable growing conditions. The bacterium is spread by wind, rain and infected budwood which can also be a method of spread. Avoid using high fertilizer rates in the late spring or early summer. This allows a tree to enter dormancy normally rather than encouraging late fall growth which is more easily infected. Prune when trees are fully dormant (January and February). Trees pruned early in the year prior to dormancy can be infected with the bacterium carried on pruning shears. Trees showing signs of bacterial canker should be left and pruned after all other trees have been completed. Early pruning encourages late fall growth of trees which is more susceptible to the bacterium. When pruning diseased trees, dip pruning equipment in a solution of 1 part household bleach and 9 parts water. At the end of the day, rinse and oil lightly to stop corrosion.
Brown Rot (fungus - Monilinia fructicola): This is a fungal disease of plums which attacks fruit in the blossom stage, as the fruit approaches maturity, and during storage. Surface moisture and moderately warm temperatures encourage its development. Fruit damaged by wind, hail, insects, or mechanical means is more susceptible to this organism. Blossom blight is the stage where blossoms are killed by the fungus. Infected blossoms are brown and water-soaked. The fungus grows down the pedicel into the stem which may cause twig dieback or a canker. During high humidity, diseased blossoms and stems may become covered with "tufts" of gray spore masses. The "tufts" are inoculum for further infection as the fruit matures. On mature fruit, the fungus enters through natural openings or wounds and rapidly develops a brown, water-soaked lesion. The organism overwinters in mummies, stem cankers and old fruit peduncles. Control is by repeated fungicide application and sanitation. Picking containers should be cleaned with a captan solution. Packing containers should be new.
Coryneum Blight of Stone Fruit (fungus - Coryneum beijerinckii): See section on Peach.
Bacterial Spot of Plum (bacterium - Xanthomonas pruni): Symptoms are observed first as small, irregularly shaped lesions. The spots are pale green in contrast to the dark green surrounding tissue. In advanced stages, angular lesions are formed and surrounded by a halo of lighter colored tissue. The inner portion of the lesion turns black and drops out at maturity. This gives the leaf a "ragged" or "shot hole" appearance. Leaves heavily infected with bacterial spot turn yellow and eventually fall. Disease first appears as a small, olive-green, circular spot on the fruit surface. However, as the disease develops, spots become darker and lesions are scattered over the fruit surface. In advanced stages of fruit infection, "pitting" occurs. "Pitting" results when bacteria kill cells in the lesion, and as surrounding healthy tissue grows, a "pit" or split is formed. This "pit" serves as an entry for other disease causing pathogens. During periods of high humidity, gum flows from the lesion which further disfigures the fruit. The area below the "pit" is corky and unpalatable. Fruit infected with bacterial spot is unsuitable for sale. Twigs are damaged by two distinct types of lesions. "Spring cankers" which develop on young, succulent twigs of the previous summer's growth appear as water-soaked, slightly darkened blisters about the same time as first leaves appear. As the season progresses, the epidermis over the lesion ruptures and releases bacteria. In time the lesion heals and becomes inactive. "Summer cankers" are restricted in size with indefinite margins. Bacteria overwinter in summer canker infected twigs. The summer cankers are the overwintering site for the bacteria. Bacteria also reproduce in lesions on the leaves and this serves as inoculum for later infection. Repeated infection occurs throughout the growing season as long as the environment is favorable for disease development. In the fall, bacteria are carried to young, succulent stems where infection is through natural openings. If the trees have stopped active growth, bacteria enter the host and undergo limited development. However, if the trees are still actively growing, the host plant responds by walling off bacteria. This results in the bacteria dying. Chemical control during the season is ineffective. Dormant sprays have been somewhat effective, the spray is accurately timed to protect the stem tissue during the fall infection period.
Rust (fungus - Tranzschelia discolor): Rust occurs on both peach and plum trees. Reddish pustules occur on the lower leaf surface marked by a yellow spot on the upper surface. It causes premature defoliation. After the crop has been harvested, make applications of a protective fungicide on a 2-3 week interval until October 1.
Cherry Leaf Spot (fungus - Coccomyces hiemalis): Infected leaves have small circular purple lesions which turn brown and eventually drop out. This gives the foliage a ragged or shot hole appearance. The fungus overwinters on old leaves. The disease is most active at 60 to 68 degrees F. Rainfall is necessary for disease development. Protective fungicides must be applied throughout the growing season to get effective control.
Black Knot on Plum (fungus - Apiosporina morbosum): Affected limbs and twigs are malformed as a result of black woody growths (galls). The galls are similar to those caused by crown gall on the root system. The disease is more prevalent on the small twigs, but under severe disease conditions it may be found on large limbs. Infection of new wood occurs from bloom until shuck split stage. Primary inoculum comes from one to two year old galls. The fungus can penetrate the uninjured epidermal stem tissue. As the fungus grows, it may develop further down the twig and form a secondary gall. Control of black knot is by sanitation. Prune and destroy the galled tissue. Make cuts two or three inches below the gall to insure complete removal of the fungus. Fungicides applied in the early bloom periods will help reduce the occurrence of this disease.
Plum Pockets (fungus - Taphrina pruni): Fruit infected with the plum pockets fungus are puffy and distorted and they fall soon after infection. The first symptom of infection is presence of small whitish spots on the plum. This happens two to three weeks after shuck split. Once infected, the flesh becomes spongy and the embroyo is killed. The skin of the fruit is first a reddish color and then a velvety gray. For this disease to develop, cool, wet conditions are necessary during early fruit development. It is similar to peach leaf curl in its dependence on weather for infection. The fungicide program used to control peach leaf curl will also control plum pockets.
Peach Stem Pitting (virus): See Peach. Peach Mosaic (virus): See Peach.
Leaf Spot (fungus - Cercospora fragariae): This is one of the more common leaf spots of strawberry. It occurs in all areas of Texas. At first, small round purple spots are seen on the upper side of the leaves. As the leaf matures, the center of the spot becomes tan or gray then almost white while the edges of the spot remain purple. On the underside of the leaf, the spots are bluish or tan in color. The fungus causing leaf spot overwinters easily in Texas since strawberries are a perennial plant in Texas, and there are always present enough infected leaves to provide inoculum. Varieties vary in their resistance to leaf spots. Chemical control can be obtained using a well planned spray program with repeated application. Sprays must be applied with enough pressure to obtain thorough coverage on the upper and lower side of the leaves.
Leaf Scorch (fungus - Marssonina fragariae): The disease is similar to leaf spot. At first, small dark purple spots up to one-fourth of an inch in diameter appear on the upper side of the leaves. These spots never have light centers as do those of leaf spot and they have a more irregular outline. At maturity, the leaf spots are large and irregular in outline and never show the white center described for leaf spot. The disease can also attack other parts of the plant, and the spots may appear on other affected parts. Sanitary measures such as raking and burning of old leaves, clean cultivation, and frequent renewal of the plants may help reduce disease losses. Some varieties have resistance to leaf scorch. Fungicide applications at repeated intervals should give adequate control. Follow recommendations as for leaf spot control.
Red Stele (fungus - Phytophthora fragariae): This is the most serious root disease of strawberries. It occurs during the late winter and spring. This fungus attacks only strawberries. The fungus is spread from one area to another primarily through diseased plants. In a healthy plant, the roots are a yellowish-white and in a diseased plant, the center is a distinctive brownish-red. Infected plants are stunted and wilt in dry weather. The root systems have a rattail appearance with few lateral roots produced. The disease is more disastrous in low, poorly drained fields during cool weather. There is no chemical or cultural methods for reducing disease losses, except to replant resistant varieties. The fungus does not persist in well drained soils. To prevent the possible build-up of a more virulent strain of the fungus, crop rotation should be practiced. Transplants should be thoroughly examined for the rattail appearance to prevent bringing in diseased plants. Avoid planting any but resistant varieties which are adapted to your area of Texas if the disease is known to occur in your garden or field.
Verticillium Wilt (fungus - Verticillium albo-atrum): This is a wide-spread soil-borne disease of strawberries occurring in cool, poorly drained soils. Symptoms show up as a burning of the outer margins of the leaves. Young roots turn black at the tips and die back. If Verticillium has been a problem in your area, consider a variety which has resistance to the wilt organism. Soil fumigation with a chloropicrin-containing fumigant is recommended when resistant varieties are not available. When using soil fumigants, follow manufacturer's directions and use caution in handling the material.
Black Root Rot (various fungi): The exact cause of black root rot is unknown. It has been associated with several different organisms or conditions. The symptoms of black root rot are a black and woody root system. On healthy plants, the roots will appear white and pliable; however, diseased plants will have a stiff root system. The feeder roots are partially or completely destroyed. The tip of the main roots will be destroyed. Fumigation has helped with this problem.
Anthracnose (fungus - Colletotrichum fragariae): The fungus attacks leaves, stolons, petioles, fruit and plant crown. Leaf infection is known as black leaf spot. Elongated brown to black lesions appear on stolons and petioles when infected. The most serious phase is crown infection which results in poor runner production and plant death. A longitudinal cut through the crown reveals a firm, reddish brown rot. Avoid excess fertility which favors disease development. Anthracnose is difficult to control with fungicides. When observed use fungicides as often as label allows.
Fruit Rots (fungi - Botrytis cinerea, Rhizoctonia spp.): fruit rots destroy a large percentage of the fruit each year in Texas. There are several organisms involved in this complex. Symptoms will be somewhat different for each disease. Gray mold rot Botrytis) is the most damaging. It attacks the green fruit as well as the ripe fruit. The first sign of infection is where the fruit has touched the ground or another diseased strawberry. It begins as a small, brown, soft spot. the spot spreads rapidly, often giving the berry a whiskered effect. To control this disease, the plants must be spaced so that the foliage or fruit does not stay wet for extended periods of time. The hard rot, caused by Rhizoctonia, attacks only ripe fruit that are in contact with the soil while growing. Affected berries are usually one sided and show a hard brown area on the side that touched the ground. Other control measures that will help reduce diseases should be used. Avoid applying heavy applications of fertilizer in the spring. Spring fertilization will cause the foliage to be dense and thus reduce the rate at which the soil dries. Spraying or dusting with fungicides will prevent harvest rots if properly applied. Spray with a fungicide as soon as the young fruit bud becomes visible and continue spraying every two weeks until the fruit is one-third grown. Additional sprays may be required if the weather is cool and wet.
Consult the Chemical Control Supplement (B-1140A) for specific chemical control suggestions.
Leaf Leaf Red Verticillium Virus
Variety Spot Scorch Stele Wilt Anthracnose Diseases
Cardinal R R S S - -
Sunrise S R R R S -
Sequoia R T S S-I R T
Tioga S - S S S T
Douglas - - S S S -
Tangi R R S S S -
Fort Laramie I I S I-R S -
Ogalla R - S - S -
Ozark Beauty R R S S S -
Pocahontas R I S S I -
Sure Crop R R R R S T
R = Resistant
S = Susceptible
T = Tolerant
I = Intermediate