MULTIPLE DISEASE RESISTANCE TO POWDERY MILDEW, BACTERIAL BLIGHT, AND ALTERNARIA BLIGHT IN LILACS (SYRINGA SPP.)

Journal of Arboriculture, Jan 2005 by Mmbaga, Margaret T, Sauvé, Roger J, Nnodu, Emmanuel, Zhou, Suping

Symptoms of powdery mildew began to appear in July (1997, 1998, and 2002) and in June (1996). In response to humidity gradients, disease symptoms first appeared on the lower leaves then progressed to upper plant parts. Powdery mildew symptoms persisted throughout the growing season and increased in severity over time to reach their highest level in August (Figure 1b). A total of 29 cultivars (S. meyeri, S. prestoniae, S. josiflexa, S. hyacinthiflora, S. reticulata, S. micmphylla, S. patula, S. henryi, and 13 cultivars of S. vulgaris) were resistant to powdery mildew; 19 cultivars of S. vulgaris and one of S. chinensis were susceptible or moderately susceptible (Table 2). Although some defoliation associated with severe powdery mildew infection (data not shown) was observed, the effect of this disease is mostly aesthetic and caused no significant difference in growth.

Beginning in mid-June to early July, severe leaf scorching caused by A. alternata developed. Scorched areas consisted of brown, necrotic lesions with ash-colored centers (Figure 1c). In some taxa, the lesions were lighter in color, with or without stem lesions. Severely infected plants defoliated, losing 90% to 95% of their leaves by late July/early August. Some of the cultivars resistant to bacterial blight and/or to powdery mildew also defoliated. Only seven cultivars showed resistance or moderate resistance to A. alternata: S. meyeri Talibin' and 'Dwarf Korean', S. vulgaris 'Mme. Antoine Buchner' and 'Silver King', S. reticulata 'Ivory Silk' and ' Summer Snow', and S. hyacinthiflora Tocahontas' (Table 3).

Twenty-four cultivars were consistently resistant to bacterial blight and powdery mildew, six cultivars to powdery mildew and Alternaria blight, and four cultivars to bacterial blight and Alternaria blight (Table 4). Only two cultivars, S. meyeri 'Dwarf Korean' and S. meyeri Talibin', were resistant to all three diseases (Table 4).

Alternaria

Alternaria alternata was constantly isolated from diseased lilacs. The fungus was cultured on potato dextrose agar, and each isolate produced identical growth patterns and conidia, Disease-free plants of the same taxon were inoculated with spore suspensions prepared from a mixture of several isolates. The resulting disease symptoms on these plants were identical to those observed on the original plants from which the isolates were obtained. Re-isolation of the fungus from these plants yielded cultures identical to the originals. Conidia were dark in color, with the typical longitudinal and transverse septa along with the characteristic long beak found in the genus Alternaria (Barnett and Hunter 1998).

The following accessions were inoculated with spore suspensions prepared from the primary cultures: S. presloniae 'Isabella', S. prestoniae 'James McFarlane', S. meyeri 'Dwarf Korean', and an unnamed S. vulgaris accession. All accessions developed identical symptoms as observed in the field. Symptom expression began to occur 6 days after inoculation and increased in severity for 10 weeks. Depending on the taxon, infected leaves developed brown, necrotic lesions with or without chlorotic bands and with or without concentric rings. Plants sprayed only with distilled water did not develop disease symptom.