Disease of the month: Two new Bipolaris diseases in South Florida

From Dragon Fruit/Pitaya Project

Bipolaris fruit rot of pitaya
Pitaya (Hylocereus undatus), or strawberry pear, is a crop that is gaining importance in south Florida. It is commonly grown as a backyard ornamental for its fleshy, sweet fruit, and is also grown commercially on an increasing amount of acreage. Recently, rotten pitaya fruit from several commercial growers and back yard plantings have been submitted to the diagnostic clinic. This rot begins as small tan, circular lesions on the fruit surface and as the diseases progresses the lesions enlarge and turn black. Usually a black felt-like growth of the fungus can be observed on the lesions. Under ideal conditions (warm and humid), the fruit develop large areas of soft rot.

Causal Agent.
The lesions are caused by a fungus, Bipolaris cactivora. This species causes stem rot of cacti in California, Florida, and Europe, and causes a fruit rot on pitahaya in Japan. The fungus is easy to culture and produces black, felted colonies on PDA. Conidia are pale to medium golden brown and curved to straight, with 2-3 pseudosepta (the septations do not extend the entire width of the spores). Bipolaris is among several genera that were formally classified as Helminthosporium, a group of pathogens that cause a variety of leaf spots on ornamental plants, especially on grasses.

From Cordyline Project

Disease Cycle and Epidemiology.
The disease is most severe on mature and ripe fruit. While young stems are susceptible to B. cactivora, mature stems are relatively resistant to infection. Small tan lesions were produced by inoculating mature, wounded stems, but the lesions did not grow larger than 5mm. In the field, flowers also exhibit lesions with dark sporulation, and may serve as an inoculum source for developing fruits. While smaller lesions are seen on green fruit, rapid lesion growth and disease development occurs on ripe fruit after harvest.

Although the epidemiology of the disease on pitaya fruit has not been studied, Bipolaris rot on ornamental cactus is most severe between 75-91 F. In general, diseases caused by Bipolaris are favored by humid conditions. Inoculum sources include diseased plants in the field and crop residue. Conidia are most often spread by wind, irrigation and rain.

Management.
Currently there are no fungicide labeled for use on pitaya in Florida. Cultural management includes limiting canopy wetness by irrigating in the morning so plant surfaces can dry quickly throughout the day. Maintain a weed free planting and remove and discard of diseased plants (i.e. stems, fruits, and flowers) promptly when symptoms occur.


Bipolaris leaf spot of cordyline

Cordyline is a popular ornamental plant in South Florida landscapes. Last summer several cordyline samples were submitted to the clinic with elongated tan lesions with a red margin. Black sporulation was visible in the center of the lesions using a dissecting microscope.

From Cordyline Project

At greater magnification, these spores were identified as a Bipolaris species. Spores produced in the lab were inoculated on healthy cordyline plants, and after one week of incubation the same lesion symptoms occured, and the Bipolaris species was recovered from the new lesions. This demonstrated that the Bipolaris species was causing the disease.

The Bipolaris sp. causing this new leaf spot produces dark brown, felted colonies on PDA. Like B. cactivora, conidia are pale to medium golden brown, and are curved with 3-8 pseudosepta. We are currently working to identify the species name of this pathogen.

We are just beginning to investigate the epidemiology of this disease. Currently we are characterizing tolerance/resistance to Bipolaris leaf spot in different cordyline varieties, as well as the effect of temperature on disease development.