GUIDE FOR IDENTIFICATION OF IMPORTANT DISEASES IN STRAWBERRY IN CALIFORNIA Anthracnose Angular Leaf Spot Leaf Blotch and Stem-end Rot Gray Mold Powdery Mildew Phytophthora Crown Rot Verticillium Wilt W. D. Gubler A. J. Feliciano H. Su Department of Plant Pathology Copyright by the Regents of the University of California Davis, California
ANTHRACNOSE CAUSED BY COLLETOTRICHUM ACUTATUM A B C Symptoms on leaves: Reddish-brown spots on leaf blade (A, arrow), and reddish-brown scorch-like symptoms at the tip (B, arrow) and/or along margins of leaves (C, arrows). Necrotic lesions may have salmon-colored spore masses under humid conditions (D, arrows). The presence of salmon-colored spore masses makes it easy to identify lesions caused by Colletotrichum from those caused by other organisms. D
ANTHRACNOSE CAUSED BY COLLETOTRICHUM ACUTATUM Symptoms on fruit: Fruit are susceptible to infection at all stages of development. Infection of immature fruit shows as dry, hard, sunken dark lesions, sometimes on the side of a fruit (A), and/or at the blossom end part of the fruit (B). These infected immature fruits usually dry and become mummified (Insert, B). Typical symptom on ripe fruit are round, blackened, sunken, firm, and dry lesions which may be covered with salmon-colored spore masses (C). A B C
ANTHRACNOSE CAUSED BY COLLETOTRICHUM ACUTATUM A volunteer plant from previous season s planting with Colletotrichum-infected fruits. Infected volunteer plants could serve as source of inoculum for infection of new plantings.
ANTHRACNOSE CAUSED BY COLLETOTRICHUM ACUTATUM Symptoms on stolons, petioles and roots: Typical symptoms of Colletotrichum infection on stolons (A), petioles (B,C), and roots (D) are presence of disk-shaped, dry, dark brown to black, sunken lesions that are sharply demarcated from the surrounding healthy tissues. When runners are girdled by lesions, the daughter plants beyond the lesion wilt and die. Lesions on petioles may also result in death of leaves. Infection of roots may result in stunting of plants. A A B C D D
ANTHRACNOSE CAUSED BY COLLETOTRICHUM ACUTATUM Symptom on crown: Internal tissues of infected crowns usually show varying degrees of reddish-brown or cinnamon-colored discoloration (right). Healthy crowns are usually cream or tan in color (left).
ANGULAR LEAF SPOT CAUSED BY XANTHOMONAS FRAGARIAE A B C Lesions caused by X. fragariae on the lower leaf surface are angular and appear dark green or water-soaked when viewed with reflected light (A) and translucent when viewed with transmitted light (B). Initially, lesions are visible only on the lower leaf surface but as the disease progresses, lesions become visible on the upper leaf surface as reddish-brown spots (C). At this advanced stage, the disease is difficult to differentiate from other foliar diseases such as Phomopsis leaf blight caused by Phomopsis obscurans, leaf spot caused by Mycosphaerela fragariae, and leaf scorch caused by Diplocarpon earliana.
ANGULAR LEAF SPOT CAUSED BY XANTHOMONAS FRAGARIAE Foliar Infection: Infection sometimes starts along the veins and appears watersoaked under reflected light (left, arrow) and translucent under transmitted light (right, arrow).
ANGULAR LEAF SPOT CAUSED BY XANTHOMONAS FRAGARIAE Infection by X. fragariae results in production of bacterial ooze under humid conditions (left) which appears as white thin film when it dries (arrows, right). Presence of white thin film on the underside of the leaf is one of the most recognizable signs of bacterial infection on strawberry leaves.
ANGULAR LEAF SPOT CAUSED BY XANTHOMONAS FRAGARIAE Calyx Infection: Lesions on calyx are identical to foliar lesions, dark green and water-soaked under reflected light (left) and translucent under transmitted light (right). Calyx infection results in direct loss to growers as it renders fruit unmarketable or lowers commercial value. This type of infection is rarely seen in California.
GRAY MOLD CAUSED BY BOTRYTIS CINEREA A B C Fruit infection: Fruit rot may start on any portion of the fruit but is most frequently initiated under the calyx and spread when fruit touch other rotten fruits or when spores are watersplashed to other blossoms or fruits. Affected tissue turns brown and becomes soft and watery (A). Rotted berries retain their general shape. Diseased portions are usually covered with gray velvety growth (B) or white mycelia which are covered with spores (C).
GRAY MOLD CAUSED BY BOTRYTIS CINEREA Leaf infection first appears as small, yellowish or tan spots which later coalesce and covers the entire leaf. Leaf blight is difficult to distinguish from other foliar diseases of strawberries at the early stage of infection. Gray mold also affects the petiole and is manifested as brown to black decay at the base of the stem.
POWDERY MILDEW CAUSED BY SPHAEROTHECA MACULARIS F. SP. FRAGARIAE Symptoms of infection on leaves: A = Appearance of white patch of mycelium with spores on the upper (left) and lower (right) leaf surface at the initial stage of infection, B = Purple to bronze blotches on the upper and lower leaf surface at the later stage of infection, C = Rolling upward of leaf edges. A A B C
POWDERY MILDEW CAUSED BY SPHAEROTHECA MACULARIS F. SP. FRAGARIAE Fruit infection: The fungus sporulates and forms profuse white mycelia on the surface of green and ripe fruits. Note heaviest growth of fungus associated with seed. Infection around individual seeds result in shrinking of the tissue resulting in a fruit with protruding seeds.
ROWN ROT CAUSED BY PHYTOPHTHORA CACTORUM hotos courtesy of Greg Browne A = A strawberry field showing plants infected by P. cactorum. B = Wilting of leaves is the most obvious symptom of infection of plants by P. cactorum. C = An infected crown showing typical chocolate brown discoloration of internal tissues. B A C
DISEASE SYMPTOMS CAUSED BY PHYTOPHTHORA FRAGARIAE Photos courtesy of Greg Browne VAR. FRAGARIAE ON ROOTS Rat-tail (arrow) appearance of diseased roots. Red steele root rot caused by P. fragariae var. fragariae. The roots have been cut open lengthwise to show the red steeles (arrows).
VERTICILLIUM WILT CAUSED BY VERTICILLIUM DAHLIAE Photos courtesy of John Duniway A strawberry production field with severe infection by V. dahliae. Nursery runner plants affected by Verticillium wilt. The older leaves are affected first, showing marginal and interveinal necrosis. The inner leaves (circled) become stunted but remain green until the plants undergo water stress.
ANTHRACNOSE CAUSED BY COLLETOTRICHUM ACUTATUM Blossom Infection: Infection may start from the flower buds, sepals, pedicels, or peduncles. If the pedicels are girdled (white arrows), individual flower buds dry and turn brown. In some cases, the peduncles are girdled (yellow arrows) and the entire flower truss wilt and die giving a blighted appearance.
LEAF BLOTCH CAUSED BY GNOMONIA COMARI (ANAMORPH ZYTHIA FRAGARIAE) Symptom of leaf blotch: A = V-shaped lesions, usually at the edge of the leaf, and usually with concentric rings of pycnidia (arrow) in the necrotic tissue, B = purplish to brownish blotches (arrows) on leaf blade.
STEM-END ROT CAUSED BY GNOMONIA COMARI (ANAMORPH ZYTHIA FRAGARIAE) Symptom on Fruit: Part of fruit (left, arrow) or whole fruit (right) turns pale red and brownish in color. In both fruits, the calyx and pedicel are also infected and discolored. On fruit, the fungus usually invades the pedicel first causing stem-end rot. A healthy strawberry fruit
STEM-END ROT CAUSED BY GNOMONIA COMARI (ANAMORPH ZYTHIA FRAGARIAE) Calyx Infection,, also known as calyx blight, is manifested as necrotic lesions on calyx (arrows) which may or may not be accompanied by fruit infection.