Identification, Pathogenicity, and Safety of Alternaria eichhorniae from Egypt as a Bioherbicide Agent for Waterhyacinth

On the basis of conidial measurements, growth characteristics, and pigmentation in culture, two of three Egyptian Alternaria spp. (isolates 3 and 5) isolated from waterhyacinth (Eichhornia crassipes) were identified as A. eichhorniae and the third, isolate 6, as A. alternata. Isolate 5 (Ae5), the most virulent, caused a severe disease, characterized by leaf blight and discrete leaflesions. The disease was initiated with either conidial or mycelial inoculum of Ae5, to yield a similar level of disease severity. Different broth media and cultural conditions were evaluated for the ability to produce highly pathogenic mycelial inoculum. Fresh potato dextrose broth, shake-culturing, and incubating cultures under diurnal light for 1 week followed by continuous darkness for an additional week were most effective. Empirically, mycelia subjected to > or < 6 s of mechanical shear in a blender were less infective than those blended for 6 s. Disease severity increased with increasing inoculum density up to 5% mycelial wet weight; a higher inoculum density did not provide greater disease severity. Exposure of inoculated leaves to at least 10 h of dew was conducive to a high level of disease, and the use of a hydrophilic mucilloid as a humectant with the mycelial inoculum augmented the disease levels. None of 97 economically important, nontarget plant species and cultivars evaluated in host range trials was susceptible to Ae5. Thus, the results of this study affirm A. eichhorniae to be an aggressive, safe, and desirable pathogen for development as a bioherbicide for waterhyacinth in Egypt.