Interrelationship of Aphelenchoides fragariae and Xanthomonas begoniae on Rieger begonia.

Disease complexes initiated by plantparasitic nematodes and bacteria on aerial plant parts are relatively uncommon. Interactions involving Aphelenchoides ritzemabosi (Schwartz) Steiner & Buhrer, Anguina tritici (Steinbuch) Chitwood, and Ditylenchus dipsaci (Kiihn) Filipjev with species of Corynebacterium are known (1, 3, 8). Ditylenchus dipsaci also is reported to interact with Erwinia rhapontici (Millard) Burkholder on aerial portions of rhubarb (6). Meloidogyne hapla Chitwood increases the severity of bacterial wilt of alfalfa caused by Corynebacterium insidiosum (McCull) Jens (4, 7), and M. incognita (Kofoid & White) Chitwood increases wilt symptoms caused by Pseudomonas solanacearum Smith in potato and tobacco (2, 5). In these wilt diseases, however, the nematodes and bacteria interact primarily on the roots. Xanthomonas begoniae (Takimoto) Dowson frequently causes a leaf blight of Rieger begonia (tuberous begonia X Begonia socotrana Hook) in Ohio. Observations suggest that this disease occurs more frequently and that symptoms are more severe in greenhouses where Aphelenchoides fragariae (Ritzema Bos) Christie, a common pest of begonias, is also prevalent. This paper reports work to determine whether an interaction exists between A. fragariae and X. begoniae on Rieger begonia. Tests were made with Schwabenland Red variety Rieger begonias (Mikkelsens, Inc., Ashtabula, Ohio). Six plants, each bearing four expanded leaves, were used for each of eight treatments. Experiments were repeated at least once. Plants inoculated with A.fragariae received 5000 nematodes/leaf placed directly on each leaf by previously described techniques (9). Leaves of plants inoculated with X. begoniae were sprayed to drip-off with 48-h-old shake cultures (108 cells/ml) maintained on nutrient broth (Difco Laboratories, Detroit, Mich.). Treatments were as follows: X. begoniae only; A. fragariae only; X. begoniae and A. fragariae inoculated within 1/2 h of each other; X. begoniae then A. fragariae 2 days later; X. begoniae then A. fragariae 7 days later; A. fragariae then X. begoniae 2 days later; A. fragariae then X. begoniae 7 days later; nutrient broth only. Plants were enclosed in plastic bags for 48 h following each inoculation step. Plants were placed in a shaded greenhouse at 21-24 C during the day and 18 C at night. Alone, X. begoniae in 3-4 wk caused symptoms at the leaf margin consisting of small, water-soaked lesions which later became necrotic (Fig. I-A). The surrounding