Robert J. McGovern

New report of Colletotrichum gloeosporioides causing postbloom fruit drop on citrus in Bermuda

Abstract Postbloom fruit drop (PFD) of citrus was observed for the first time after a widespread and severe outbreak occurred in Bermuda in the 1990s. Fruit losses from the disease were estimated at 25% to 35% in sweet orange, grapefruit, lemon and Tahiti lime. The causal agent of PFD has been reported as either Colletotrichum gloeosporioides or Colletotrichum acutatum. Bermuda isolates of Colletotrichum recovered from diseased orange trees produced lesions typical of PFD in detached petals of orange, lime and grapefruit, and in attached orange blossoms. The isolates produced few to no lesions of anthracnose on leaves of key lime seedlings. Kochs postulates were fulfilled following reisolation of morphologically identical fungi from inoculated tissues. The isolates were characterized using morphological (conidial size and shape and colony colour), physiological (growth rate at 24 °C and on a benomyl-amended medium), immunological (ELISA) and molecular (PCR amplification and sequencing of the ITS region) methods. Immunological and molecular techniques provided definitive identification of the isolates as Colletotrichum gloeosporioides. This report confirms that C. gloeosporioides can be the causal agent of PFD; therefore, identification of the causal agent to species by immunology, molecular analysis and fungicide sensitivity is suggested for new outbreaks of the disease.

Evaluation of Irradiated Caribbean Fruit Fly (Diptera: Tephritidae) Larvae for Laboratory Rearing of Doryctobracon areolatus (Hymenoptera: Braconidae)

ABSTRACT We report here that it is possible to rear D. areolatus on irradiated A. suspensa larvae without adversely affecting sex ratio and overall parasitoid emergence and with no adult A. suspense emergence. There was no difference in emergence of D. areolatus adults from irradiated versus non-irradiated hosts (72.4 ± 1.9% vs. 73.0 ± 1.9%), and no difference in sex ratio of parasitoids obtained from irradiated and non-irradiated hosts (50.0 ± 1.6 and 47.0 ± 1.4% female, respectively). The successful use of A. suspensa larval hosts can greatly ease the process of rearing, transporting, and releasing fruit fly parasitoids while eliminating the need to separate flies from parasitoids. Further improvements in the laboratory rearing process of D. areolatus, including irradiating late A. suspensa larvae at a lower dosage and irradiating A. suspensa as egg or early instars, are discussed.

Epidemiology and Management of Fusarium Wilt of China Asters

The epidemiology and strategies for management of Fusarium wilt of China aster (Callistephus chinensis) were studied in Connecticut and Florida, USA, by examining seed contamination, on-farm disease incidence, sanitation, host resistance, and various soil treatments. Five out of 25 commercial seed packages from three separate distribution companies assayed in Connecticut had seeds contaminated with the pathogen Fusarium oxysporum f. sp. callistephi. Farm surveys of two cut-flower farms in Connecticut had disease incidences of 32 and 58%, while in Florida, the incidence of the disease ranged from 0.002 to 71.2% in two cut-flower operations. All pathogenic isolates from seed and symptomatic plants in Connecticut were vegetatively compatible, suggesting a common origin. Pathogenic isolates from Florida and nonpathogenic isolates fell into different vegetative compatibility groups and may have had another origin. Sodium hypochlorite solutions (1%) eliminated the fungus from seeds and Styrofoam when applied as a soak or spray, respectively. Soil fumigation with methyl bromide + chloropicrin, 1,3-dichloropropene + chloropicrin, or metam sodium maintained Fusarium wilt at low levels at a Florida cut-flower production facility. Evaluations of disease resistance of 44 cultivars in the greenhouse identified eight cultivars with moderate resistance. Four cultivars were identified with moderate resistance in field trials and thus could serve as a source of resistant germplasm for future breeding programs. These findings should encourage growers to use sanitation protocols to prevent entry of the pathogen into their fields and to choose commercially available cultivars that have moderate resistance.

Effects of inoculum density, leaf age, moisture, temperature, and wetness duration on black streak of edible burdock

Inoculum density, temperature, leaf age, and wetness duration were evaluated for their effects on the development of black streak (Itersonilia perplexans) on edible burdock (Arctium lappa L.) in a controlled environment. The effect of relative humidity (RH) on ballistospores production by I. perplexans was also evaluated. Symptoms of black streak on leaves increased in a linear fashion as the inoculum density of I. perplexans increased from 102 to 106 ballistospores/ml. Rugose symptoms on young leaves were observed at densities of ≥104 ballistospores/ml. Disease severity of I. perplexans in relation to leaf age followed a degradation curve when the leaves were inoculated with ballistospores. Disease severity was high in newly emerged leaves up to 5 days old, declined as leaf age increased to 29 days, and was zero when leaf age increased from 30 to 33 days. Disease development of edible burdock plants exposed to ballistospores of I. perplexans was evaluated at various combinations of temperature (10°, 15°, 20°, 25°C) and duration of leaf wetness (12, 24, 36, 48, and 72 h). Disease was most severe when plants were in contact with the ballistospore sources at 15° or 20°C. The least amount of disease occurred at 25°C regardless of wetness duration. Ballistospores required 24–36 h of continuous leaf wetness to cause visible symptoms by infection on edible burdock. Ballistospores production in infected lesions required at least 95.5% RH.

Detection of a Severe Isolate of Impatiens Necrotic Spot Virus Infecting Lisianthus in Florida

In May 1997, inclusions typical of a tospovirus were visualized by light microscopy in leaf tissue of lisianthus (Eustoma grandiflorum) exhibiting stunting, necrotic ringspots, leaf distortion, and systemic necrosis. Wilting and plant death were the final symptoms observed. Affected plants occurred at low incidence (<0.1%) in greenhouse-grown lisianthus in Manatee County, FL. Symptomatic tissue tested positive for impatiens necrotic spot virus (INSV) and negative for tomato spotted wilt virus (TSWV) with enzyme-linked immunosorbent assay (ELISA; Agdia, Elkhart, IN). Mechanical transmission of the virus to lisianthus and tomato was attempted by triturating 1 g of symptomatic leaf tissue in 7 ml of a buffer consisting of 0.01 M Tris and 0.01 M sodium sulfite, pH 7.3. Six plants of lisianthus cv. Maurine Blue and three of tomato (Lycopersicon esculentum) cv. Lanai at the second true-leaf stage were inoculated following abrasion of leaves with Carborundum. An equal number of controls were inoculated with buffer alone. Plants were maintained in a controlled environment chamber with a 12-h photoperiod, day/night temperatures of 21/16°C, and light intensity of 120 μE · s-l · m-2. Transmission rates were 100 and 0% to lisianthus and tomato, respectively. Chlorotic local lesions followed by chlorotic ringspots were observed in inoculated lisianthus leaves 4 days after inoculation. Stunting, leaf distortion, and necrotic ringspots appeared in noninoculated leaves of lisianthus plants within 3 to 4 weeks after inoculation. Buffer-inoculated lisianthus and all tomato plants remained symptomless and tested negative for INSV by ELISA. All symptomatic lisianthus tested positive for INSV by ELISA. The symptoms we observed in lisianthus due to infection by INSV were more severe than those previously reported in this host (1,2). The occurrence of such strains of INSV at high incidences could pose a significant threat for commercial lisianthus production. References: (1) M. K. Hausbeck et al. Plant Dis. 76:795, 1992. (2) H. T. Hsu and R. H. Lawson. Plant Dis. 75:292,1991.

Diseases of Tulip

Tulip is the most important ornamental geophyte worldwide, grown for bulbs, cut flowers, potted plants, and landscape use. Major increases in consumer demand in the first half of the twentieth century were associated with the identification of many new disease problems. Tulip is susceptible to a number of diseases caused by fungi, bacteria, viruses, and nematodes including Botrytis tulipae, Fusarium oxysporum f. sp. tulipae, Pectobacterium carotovorum, Tulip breaking virus, and Ditylenchus dipsaci that can significantly reduce flower and bulb production. Since the plant is propagated vegetatively, this factor can facilitate the spread of disease if pathogen-free propagative material is not used and integrated disease management is not followed.