Apple Blotch (fungus – Phyllosticta solitaria): Shiny, black blotches develop on fruit. Edges of lesions are irregular with radiating outgrowths. Foliage symptoms are characterized by light gray spots with a dark dot in the center. Elongated, sunken, light colored lesions with black dots are formed on veins, midribs and petioles of the leaves. The spread of blotch is favored by temperatures of 75oF to 80oF and heavy rains. Several fungicides will control this disease.
Bitter Rot (fungus – Glomerella cingulata): Symptoms appear first as small, circular, brown spots on both apples and pears. The lesion enlarges and becomes saucer-shaped. The fungus overwinters in mummified fruit and cracks or crevices in bark. Most infections can be traced to a broken limb. Infection normally shows up in mid to late summer. Young fruit are resistant to the fungus. Maximum infection occurs at 85oF and after light rain. Orchard sanitation and a regular spray program are extremely important. Broken limbs should be cut to prevent overwintering of spores in these protected areas.
Black Rot (fungus – Physalospora obtusa): Black rot is a firm, textured rot. The spot is light brown and darkens with age. As it enlarges, it is marked with concentric dark bands. The lesion is convex in shape, yet in very advanced stages becomes somewhat depressed. On the leaf the lesion is a purple speck that enlarges and eventually has a center which is brown or yellowish-brown. The fungus infects foliage at petal fall. Fruit infection is not normally evident until midsummer. The disease is favored by temperatures of 80oF and light rain. Although fungicides help, sanitation is the best means of reducing this disease.
Bot Rot (fungus – Botryosphaeria ribis): The fungus infects woody tissues and fruits. Wounded apple wood is very susceptible. On limbs and twigs, infection is visible in July. It appears as a blister on the infected limbs. At maturity the blister ruptures and a liquid spreads over the limbs. The wet area then becomes sunken with a dark colored appearance. The spot spreads into the cambium where it enlarges in concentric rings until fall. Lesions stop enlarging in the fall and cracks appear along the edge of the spot. It may or may not begin growth next spring. Fruit infection is observed as small reddish-brown spots. These eventually coalesce, causing a soft rot to occur. As the decay develops, the lesion turns light brown to dark brown and is covered with beads of exudate on the surface of the decayed fruit. The fungus overwinters in live and dead limbs. In the spring, an ooze is formed which is spread over the tree. Mature and ripened fruit are very susceptible. The disease develops at temperatures of 75oF or above. Botryosphaeria infection is encouraged by any condition which reduces tree vigor such as drought, winter injury, and low nutrition. Jagged pruning wounds and grafting sites are sometimes invaded. Pruning out the decayed areas and a thorough spray program will significantly reduce losses to this disease.
Cedar Apple Rust (fungus – Gymnosporangium juniperi-virginianae): The fungus passes part of its life on apple and part on the cedar. On cedar it forms reddish galls which swell and produce an orange slime each spring. On apple it causes spots on leaves, fruit and sometimes on young twigs. The spots on the foliage and fruit are orange-yellow in color, and a blister or cushion forms in the center of each spot. Where this disease is a problem, cedars should be destroyed for 1 mile around an apple orchard. Normal spray programs will control infection on apples. Resistant varieties should be used when available. Golden Delicious and Delicious are resistant. Most ornamental Crabapples and Jonathans are susceptible.
Chlorosis: (See iron deficiency)
Cotton Root Rot (fungus – Phymatotrichum omnivorum): Apples should never be planted in soil infected with this fungus due to its extreme susceptibility. See the separate section on Cotton Root Rot for more information.
Crown Gall: (See section on Crown Gall)
Fire Blight (bacterium – Erwinia amylovora): This bacterium causes twig dieback and is common on apples and pears as well as some 75 other plant species. Blossom blight is common on apples. Infected blossoms wilt and turn black rapidly. The foliage does not immediately abscise due to the rapid rate of death. Twig blight is common and can result in dieback ranging up to 12 to 36 inches of twig length. The bacteria overwinters in cankers. Bacteria oozes from the cankers in early spring and are carried by wind and insects to healthy foliage. Resistant varieties should be used when available. Fungicides containing copper will cause russetting on apples. Trees which have had fire blight should be pruned to remove overwintering cankers. Nitrogen levels should be kept low. No organic nitrogen sources should be used as they encourage continual growth which is susceptible to the bacterium. Make cuts from 4 to 6 inches below the visible cankers. All pruning equipment should be sterilized with a one part household bleach to nine parts water solution. For more information, refer to the pear section.
Fly Speck (fungus – Leptothyrium pomi): Numerous small, shiny, black spots are formed on the fruit. Control is the same as sooty blotch.
Mushroom Root Rot (fungus – Armillaria mellea): Mushroom root rot is caused by a soilborne fungus which attacks apples and causes a slow decline. Trees may be infected with the fungus for 2 to 3 years before death occurs. The vigor of the tree is noticeably reduced during this time. Trees infected with Armillaria mellea have a layer of creamy white fungal growth between the bark and the wood at the soil line. The growth (rhizomorphs) girdles the trunk and the tree dies when this girdling is complete. Trees damaged by the fungus can be diagnosed by the presence of the rhizomorphs and the advanced stage of decay of the root system. Infected trees are loose in the soil due to the deterioration of the root system. Mushroom root rot is most often found in areas where post oaks were the predominate native timber and in sandy soils. This does not always hold true, and growers must be aware that the disease can cause serious losses in many soil types should it become established. Soil fumigation will give only temporary control of this problem. The wide distribution of the fungus in the soil (both vertically and horizontally) and the constant recontamination of a fumigated area from non fumigated soils reduce the early advantage gained by soil fumigation. Resistant root stocks are not available. Growers should avoid planting in areas where the fungus has been a problem in the past or on recently cleared post oak timberland. It is best for a homeowner to move to another planting site should plants die from mushroom root rot. See section on Mushroom Root Rot.
Phytophthora Collar Rot (fungus – Phytophthora cactorum): Black, slimy cankers are formed from the bud union to the lower portion of the trunk. Trees infected with this fungus are stunted and eventually die. It is particularly damaging on the Malling Merton Clones. Most seedlings of Delicious, Melba, McIntosh, and Wheatly are resistant. No chemical control recommended. See section on resistant varieties.
Powdery Mildew (fungus – Podosphaera leucotricha): Young foliage shows cupping and twisting. The leaves are covered with a white, powdery mass of spores. Net russetting will occur on fruit when it is infected with this fungus. All varieties are susceptible, yet Jonathan appears to be more susceptible than most. Use of a regular fungicide program will control this problem. Karathane, a commonly recommended fungicide, should not be used at temperatures of 90oF or above. A spreader should be used to improve control.
Quince Rust (fungus – Gymnosporangium clavipes): This fungus attacks both apple and pear. It attacks the blossom end of the fruit and restricts the growth of the apple. Tissue below the lesion dies and becomes spongy. Quince rust, like cedar apple rust, must complete part of its life cycle on cedar. Apple varieties susceptible to quince rust include Delicious types. Use fungicides and remove cedars in the area for control.
Scab (fungus – Venturia inaequalis): This fungus attacks both the foliage and the fruit. It first appears on the leaves as small, dull, smoky areas which with age become olive colored, velvety, and much more visible. On fruit, the lesions are first small, circular, olive green areas which turn dark and scabby. The spots crack open in late stages of development. The fungus overwinters in fallen apple leaves. The fungus produces fruiting structures in the spring. Rains cause spores to be shot into the air where they are carried to developing foliage. The spore will germinate and produce a leaf spot if moisture is present when the spore lands on the leaf. Further spread occurs from these spots to adjoining leaves of fruit. Apple scab is dependent on moisture for development.
Sooty Blotch (fungus – Gloeodes pomigena): Sooty-gray or cloudy blotches appear on the fruit. The fungus attacks both apples and pears in Texas. It is most severe during years that have cool, wet springs, late summer rains, and low temperatures in early fall. It is normally found in conjunction with fly speck, another disease of apples and pears. Fungcides should be applied in late spring and early summer.
Southern Blight (fungus – Sclerotium rolfsii): Infected trees die rapidly (2 to 3 weeks) after the first visible symptoms are observed. The leaves remain attached to the tree. Creamy white to yellow rhizomorphs are formed on the outside of the roots. As the growth develops, small, dark brown sclerotia are formed among the rhizomorphs. In Texas it has been observed attacking apple trees both in the nursery and in commercial orchards. Currently, no chemicals are cleared for the control of Southern Blight on apples. Deep burial of organic material prior to planting and controlling weeds at the base of the tree will help reduce losses. Sclerotium rolfsii is a facultative saprophyte which grows on organic material in the soil, but under certain conditions, it can attack and parasitize healthy growing plants. Crop residue should be pulled away from the base of the apple trees.