Inhibition of the lemon brown rot causal agent Phytophthora citrophthora by low-toxicity compounds

BACKGROUND

Phytophthora spp., soil-borne oomycetes, cause brown rot (BR) on postharvest lemons. The management of this disease is based on cultural practices and chemical control using inorganic salts of limited efficacy. In the search for new alternatives, the aim of this work was to evaluate the effect of low-toxicity compounds to inhibit the growth of P. citrophthora and to control BR disease on lemons. Sodium bicarbonate, potassium sorbate, polyhexamethylene guanidine, Ascophyllum nodosum extract and a formulation containing phosphite salts plus A. nodosum (P+An) were evaluated.

RESULTS

All tested products inhibited mycelial growth, sporangia formation and zoospore germination of P. citrophthora in vitro. In postharvest applications on artificially inoculated lemons, only P+An exhibited a BR curative effect, with incidence reduction of around 60%. When this formulation was applied in field treatments, BR incidence was reduced by 40% on lemons harvested and inoculated up to 30 days post application.

CONCLUSION

Our results demonstrate the in vitro direct anti-oomycete effect of low-toxicity compounds and the in vivo efficacy of P+An formulation to control BR, encouraging the incorporation of the latter in the management of citrus BR. © 2020 Society of Chemical Industry.

First Report of Macrophomina phaseolina Causing Strawberry Crown and Root Rot in Northwestern Argentina

The winter strawberry (Fragaria ananassa Duch.) industry in subtropical regions is characterized by early-season fruit production, a time when crop value is the highest. This is the case of the northwestern province of Tucuman (27°S), Argentina's largest winter strawberry growing area (2). Collapsed and dying strawberry plants of cv. Camarosa were observed in commercial farms in Tucumán in September 2007 with an incidence of 16%. Crowns and roots of affected plants revealed dark, external and internal browning tissue with several dark, oblong, subepidermal sclerotia. Pieces of necrotic crowns and roots disinfested with 0.6% chloride for 30 s were placed on potato dextrose agar (PDA). After 4 days of incubation at 25°C with constant fluorescent lighting, dark cultures with numerous, dark, oblong sclerotia immersed in the isolation medium were observed. Sclerotia size ranged from 70 to 120 μm in 7-day-old cultures. The isolated fungus was identified as Macrophomina phaseolina (3). Ostiolate pycnidia bearing ellipsoidal, hyaline conidia were rarely observed on the affected host tissue (1). Pathogenicity tests were performed on micropropagated nursery plants grown in sterile potting soil under greenhouse conditions for 3 months. Single-sclerotia culture was grown in PDA at 25°C for 4 days. Then, sterile wooden toothpicks were placed on the medium for seven additional days to allow fungal colonization. Prior to use, toothpicks were autoclaved twice in deionized water and once in V8 juice (4). Four plants of Camarosa, Camino Real, Pajaro, and Chandler cultivars were inoculated by inserting a colonized toothpick into the crown. Sterile toothpicks were inserted into crowns of control plants. All plants were incubated under greenhouse conditions. The experimental setting was a randomized complete design with two replicates per cultivar. Inoculated plants developed wilting in one or more leaves 6 days after inoculation. Most of the inoculated plants (90%) collapsed or died within 2 weeks, while control plants remained healthy during the observation period. The reisolated fungus from affected plants was morphologically and culturally identical to the inoculated isolates. M. phaseolina has been reported in the province of Santa Fe (~32°S) where winters are humid and cold (1), contrasting with Tucumán winters, which are dry and cold, both highly conducive conditions for the development of the disease. To our knowledge, this is the first report of M. phaseolina, an emerging pathogen, considered as a potential threat for northwestern Argentina's strawberry industry, which is recognized as a major source of employment and income in this region. References: (1) C. J. N. Carrera. Ficha Fitopatológica 000867 INTA IMIZA Castelar Bs.As., 1972. (2) D. S. Kirschbaum and J. F. Hancock. HortScience 35:807, 2000. (3) J. Maas. Page 26 in: Compendium of Strawberry Diseases. 2nd ed. J. L. Maas, ed. The American Phytopathological Society, St. Paul, MN, 1998. (4) J. Mertely et al. Plant Dis. 89:434, 2005.

First Report of Dothichiza caroliniana Causing Double Spot on Highbush Blueberry in South America

A foliar disease affecting Jewell, Emerald, and O'Neal cultivars of highbush blueberry (Vaccinium corymbosum L.) was observed in commercial plantings in Tucuman, Argentina during February 2007. Typical symptoms were circular leaf spots that were 10 to 15 mm in diameter, light to dark brown with dark reddish borders, and frequently exhibiting a secondary necrotic area around the original spot. The final appearance was a large necrotic area surrounding a pale, smaller spot. On the abaxial leaf surfaces, white mycelia grew from the border of the initial or enlarged spots toward the healthy tissues. Frequently, black pycnidia bearing an amber rubbery cirrhi were observed. Fewer pycnidia were observed on the adaxial leaf lesions. The fungus was isolated by transferring cirrhi from affected leaves onto petri plates containing potato dextrose agar (PDA). Plates were incubated in darkness at 27°C and light brown concentric colonies with an aerial, effuse, white mycelium developed. After 5 days, dark brown cirrhi emerged from pycnidia in culture. Pycnidiospores were obtuse, hyaline, 1-celled (7 × 2 μm), and grew from the base of pycnidia on short conidiophores. Pycnidia were black, conical, obovoid, averaged 110 × 70 μm wide, and were lightly immersed in the mycelium. Pathogenicity tests were performed by spraying a 10⁶ conidia/ml suspension on leaves of nine blueberry plants (cv. Jewell). Plants were incubated in a moist chamber at 25 to 29°C with a 12-h photoperiod. After 5 weeks, all inoculated leaves had foliar symptoms similar to the lesions previously observed in the field. Reisolation of the fungus completed Koch's postulates. Noninoculated leaves did not develop foliar symptoms. Morphological and cultural characteristics of the fungus conformed to the descriptions of Dothichiza caroliniana (Demaree & M.S. Wilcox) reported to cause "double spot" in blueberry (1,2). Symptoms were consistent with those described for double spot disease (2). To our knowledge, this is the first report of D. caroliniana on blueberry in South America. References: (1) J. B. Demaree and M. Wilcox. Phytopathology, 37:490, 1947. (2) R. D. Milholland. Double spot. Page 17 in: Compendium of Blueberry and Cranberry Diseases. F. L. Caruso and D. C. Ramsdell, eds. The American Phytopathological Society, St. Paul, MN 1995.