Angular leafspot (Xanthomonas ftagarlae ) is the only bacterial disease of importance found on strawberry. Although originally a southern disease it is now found in most areas of northern strawberry production. Infected planting stock has probably been the main route of this disease into these areas.
Leaves which are infected in the fall overwinter the bacterium and serve as a source of disease for new infections in the spring. Under our climatic conditions these leaf infections are relatively inconspicuous and do not cause significant damage. The leafspots are small and restricted by the veins of the leaf giving them an angular appearance. On the top surface of the leaf they appear as small red or brown flecks, while on the bottom leaf surface they may appear as dark green water-soaked areas. Under wet conditions the bacteria multiply quickly and can appear as very small droplets of sticky ooze associated with these spots. The best way for producers to identify this disease is to collect leaves and hold them up to the light. Newly infected areas of the leaf will be translucent yellow or light green (Figure A ). Older infections are more opaque and resemble the infections caused by common leafspot and leaf scorch. If wet conditions persist, many separate infections may join together causing the entire leaflet to darken and collapse.
Bacteria produced in leafspots are spread under wet conditions to the bloom and this is where the greatest economic damage occurs. Flowers may be killed outright (Figure B) or sections of the hull may become infected and turn black (Figure C ). For many producers the first indication that a field is infested with angular leafspot is when otherwise normal fruit are harvested which have dark or black hulls.
Ideal conditions for this disease, in the period up to and during bloom, are wet cool days with nighttime temperatures near zero. When these conditions, which are common to our production area occur, losses can be significant. Under the hot, dry conditions of mid-summer the disease becomes harder to detect, but leafspots: are easily found again in September and October.
Effective control measures for angular leafspot are limited. All currently recommended varieties are considered to be susceptible. Since the disease can be carried on transplants, producers should purchase planting stock from a nursery which is believed to be free of this disease. Production fields suspected of being infested should be scouted when buds begin to extend from the crown and again at 10 percent bloom. If angular leafspot is detected on the leaves, producers should cease irrigation or irrigate only under excellent drying conditions so that the length of time the bloom remains wet is minimized. Fungicides applied for control of fruit rot and powdery mildew will not control angular leafspot since it is a bacterial disease. Repeated applications of fixed copper applied for the control of common leafspot may give some suppression of angular leafspot if applied prior to infection periods. For more information producers should consult the Atlantic Provinces Strawberry Protection Guide.