larly coalescent with characteristic feathery, radiate, fibrillose margins of subcuticular mycelial strands (Fig. 10). In some in stances, the feathery lesion margin may be indistinct. Small black acervuli are often visible on the surface (Figs. 11 and 12) and may be distributed irregularly or in concentric circles. Conidia may be visible as white, slimy masses on the acervuli. Leaf tissue surrounding the spots turns yellow, and chlorosis extends throughout the leaflet until abscission occurs. The pathogen is actually present only in the lesion itself; the yellow tissue is caused by pathogen metabolites. The yellow tissue ex hibits a high level of metabolic activity that is expressed by the accumulation of phenolics and ortho-dihydroxyphenols and amino acids, as well as by a high level of enzymatic activity. Spots enlarge slowly, taking several weeks to reach 12 mm in
diameter. In resistant cultivars or under unfavorable environ mental conditions, only tiny, black flecks may form and leaves may not turn yellow or abscise. Yellowing (Fig. 13) and abscission of leaflets are associated with ethylene. Leaves with black spot produce large amounts of ethylene; production decreases as leaves become yellow and ceases when leaves turn brown. Infected leaves con tain less auxin than do healthy ones. The pathogen degrades this abscission-retarding material, thereby hastening leaf abscission. Raised, purple-red, irregular blotches develop on the im mature wood of first-year canes of susceptible cultivars (Fig. 14). Spots later become blackened and blistered; they contain acer vuli but lack fibrillose strands. Lesions are often small and rarely kill branches but are extremely important in the survival of the pathogen over the winter. Petioles and stipules may have inconspicuous black spots similar to those found on leaves. Petioles may be girdled with out abscising. Peduncles, fruits, and sepals may have similar symptoms. Petals may have red flecks accompanied by moder ate distortion. Acervuli frequently occur in the lesions.
Causal Organism
Marssonina rosae (Asteroma rosae Lib., Actinonema rosae (Lib.) Fr., and Marssonia rosae Trail), the anamorph stage of the black spot pathogen, was described in France in 1827. The teleomorph stage, Diplocarpon rosae, was described in New York in 1912. The pathogen is quite host specific and ap proaches obligate parasitism. Pathogenic races of the fungus are reported.
Fig. 10. Early lesions on a leaf affected by black spot, caused by Diplocarpon rosae.
Fig. 12. A close-up of acervuli of Diplocarpon rosae, the cause of black spot, in a lesion on a leaf.
Fig. 11. Acervuli of Diplocarpon rosae, the cause of black spot, in a lesion with feathery margins.
Fig. 13. Severe symptoms of black spot, caused by Diplocarpon rosae. Note the lesions with yellowing from the formation of ethylene.
9