Plum, Flowering-Brown Rot Blossom Blight

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Cause The fungi Monilinia fructicola and M. laxa can incite both a blossom blight, a twig and branch dieback. Fungi survive year to year on infected twigs, branches, old flower parts, or mummified fruit of other Prunus spp. Conidia are produced on infected plant debris in the tree when the temperature is above 40°F. Wind and rain blow spores (conidia and ascospores) to healthy blossoms in spring to begin the infection process during wet weather. Infection does not occur below 50°F and will occur for M. laxa above 55°F. Flowers can be blighted any time floral tissue is exposed but are most susceptible at full bloom. More spores can be produced on this tissue, initiating several more disease cycles during the spring. Under severe conditions, non-flowering shoots or leaves can be infected directly.

Both fruiting and ornamental plums (purple leaf plums), peaches, nectarines, prunes, cherries, almonds, and apricots are susceptible. Susceptible cultivars include Prunus x cistena, P. x blireiannna while resistant cultivars include P. cerasifera 'Frankthrees' (Mt. St. Helens), Krauter Vesuvius, Newport, and Thundercloud. During the cool wet spring of 2010 cultivars Krauter Vesuvius and Thundercloud were badly affected in western Oregon.

Symptoms Infected flower parts turn light brown and may develop areas of buff-color (M. fructicola) or gray (M. laxa) spores. Infected petals may look water soaked, which can be mistaken for frost injury. Flowers generally collapse as the fungus invades through the pedicel. Infected flowers often adhere to twigs and spurs through harvest or even winter. Depending on the fungus and plant infected, the disease may continue into twigs or spurs. Lesions may remain discrete or girdle the twig, causing all distal portions to die. Profuse gumming also may occur in these areas. Again, buff or gray spores (in sporodochia) may develop on these necrotic twigs.

Cultural control These must be supplemented by chemical control methods especially in the wettest areas such as west of the Cascade Range.

  • Remove and destroy infected twigs and branches in summer.
  • Use moderate amounts of nitrogen fertilizer.
  • Plant resistant cultivars.

Chemical control Do not use on edible fruit or within 12 months of harvest. Only one (1) or two (2) applications during bloom are needed most years. Tank-mix and/or alternate products with different modes of action to prevent the build-up of resistant fungi. Limit the use of any one group during crop production.

  • Banner MAXX at 2 to 4 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
  • Broadform at 4 to 4 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
  • Chipco 26019 FLO at 1 to 2.5 quarts/100 gal water. Group 2 fungicide. 12-hr reentry.
  • Compass 50 WDG at 2 to 3 oz/100 gal water. Do not use organosilicate additives. Group 11 fungicide. 12-hr reentry.
  • Daconil Weather Stik at 1.4 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
  • Decree 50 WDG at 0.75 to 1.5 lb/100 gal water. Group 17 fungicide. 12-hr reentry.
  • Miravis at 5.1 fl oz/A. Can be used up to the day of harvest. Group 7 fungicide. 12-hr reentry.
  • Miravis Duo at 13.6 fl oz/A. Can be used up to the day of harvest. Group 3 + 7 fungicide. 12-hr reentry.
  • Mural at 3.5 oz/100 gal water. Can be phytotoxic to certain apple and cherry cultivars. Group 7 + 11 fungicide. 12-hr reentry.
  • Pageant at 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
  • Problad Verde at 18.1 to 45.7 fl oz/A. Reapply if rain occurs within 12 hours of original application. Do not use within one day of harvest. Group BM01 fungicide. 4-hr reentry.
  • Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
  • Thiophanate-methyl-based products. Group 1 fungicides. 12-hr reentry.
    • Cleary's 3336 EG at 12 to 16 oz/100 gal water.
    • OHP 6672 4.5 F at 10.75 to 20 fl oz/100 gal water.

Reference Pscheidt, J.W., and Byther, R. S. 2001. Chapter 72. Prunus Diseases. p. 317-325 in Diseases of Woody Ornamentals and Trees in Nurseries., R. Jones, and M. Benson, (eds). St. Paul, MN: APS Press