Outbreaks of Stem and Leaf Blight of Eustoma grandiflorum Caused by a Phomopsis sp. in Florida

Between November 1997 and May 1998, numerous lisianthus (Eustoma grandiflorum) cultivars exhibited severe stem and leaf blight at two pot-flower production sites in Hillsborough and Dade counties, FL. Blight occurred in mature plants and ranged in incidence from 3 to 5% in Dade County and from 40 to 80% in Hillsborough County. Initial stem necrosis was rapidly followed by leaf blight and production of numerous dark pycnidia in diseased tissue. As stem blight progressed, infected plants collapsed and died. Pycnidia contained biguttulate, 7.2 × 2.2-μm spores typical of Phomopsis alpha conidia; beta conidia were not observed. A Phomopsis sp. was isolated consistently when pycnidia from symptomatic stems were placed on acidified 25% potato dextrose agar after surface-disinfestation in 0.5% NaOCl; only alpha conidia were observed in culture. Pathogenicity was confirmed using a suspension of hyphae, pycnidia, and conidia made by comminuting one 95-mm-diameter acidified carnation leaf agar plate containing a 4-week-old colony of the Phomopsis sp. in 100 ml of deionized water. Six plants of lisianthus 'Maurine Blue' (three wounded at the crown, three nonwounded) each were inoculated with 10 ml of the fungal suspension. An equal number of noninoculated lisianthus (three wounded, three nonwounded) served as controls. After inoculation, plants were maintained in a greenhouse with average high and low temperatures of 38 and 25°C, respectively. Stem and leaf blight symptoms were observed in two of three wounded plants and in all nonwounded plants within 11 and 15 days after inoculation, respectively. Infection by a Phomopsis sp. was confirmed by reisolation from symptomatic tissue. Although this Phomopsis sp. has been detected previously in lisianthus exhibiting leaf and stem lesions (1), this report establishes the ability of the of the fungus to act as a primary pathogen and to cause extensive losses in this crop. Reference: (1) Alfieri et al. 1994. Diseases and Disorders of Plants in Florida. Bull. No. 14. Division of Plant Industry, Gainesville, FL.