Variation in resistance mechanisms to the green peach aphid among different Prunus persica commercial cultivars.

ABSTRACT Peaches and nectarines are frequently attacked by the green peach aphid Myzus persicae (Sulzer), with significant negative impacts on fruit production. The genetic variability of resistance to this aphid among commercial cultivars of Prunus persica (L.) Batsch and Prunus persica variety nectarina was evaluated in this study. In total, 16 cultivars of P. persica were selected to evaluate the occurrence and population growth rate of M. persicae in commercial orchards, as well as in no-choice and probing behavior laboratory assays. The results showed variability between cultivars in resistance and susceptibility to M. persicae, with three cultivars exhibiting different signatures of resistance. The peach cultivar 'Elegant Lady' exhibited a low occurrence of aphids in the orchard, a low rate of growth, moderate leaf-rejection in a no-choice test and a higher number and longer period of salivation into sieve elements, suggesting resistance at the phloematic level. The nectarine cultivar 'August Red' also exhibited low aphid occurrence in the orchard, a low rate of growth, and resistance at the prephloem and phloem levels. Finally, the nectarine 'July Red-NS92' exhibited a low occurrence of aphids in the orchard, a higher number of rejections in no-choice assays and no ingestion of phloem during the probing behavior experiments, suggesting prephloematic resistance. The rest of the cultivars studied exhibited clear susceptibility. Hence, different resistance mechanisms are apparent among the studied cultivars. The information gathered in this study regarding the resistance to M. persicae may assist breeding programs aimed at increasing aphid resistance to peaches and nectarines.

Pseudomonas corrugata Causing Pith Necrosis on Tomato Plants in Baja California Sur, México.

Fresh market tomato (Lycopersicon esculentum Mill.) cultivars are grown in field and greenhouse areas in Baja California Sur from October to June for international markets. During March and April 2001, field-grown tomato plants showing external necrotic stem lesions and hollowed necrotic pith were observed in a 50-ha field 30 km south of La Paz. The average disease incidence in the field was 3%. Most infected plants presented necrotic lesions in the main stem 20 to 30 cm above the soil line. A few plants also presented necrotic lesions in lateral branches. Transversally cut sections in the necrotic stem area showed rotting of the vascular system with hollow cavities, typical symptom of pith necrosis. To isolate the pathogen, 5-cm-long transverse portions of diseased stems were excised, washed with soap and brushed, and rinsed with tap water. The stem portions were soaked in 10% bleach for 2 min, blotted dry on sterile paper, and 1-cm2 sections were cut to include the margins of the necrotic pith. These sections were plated on nutrient agar and incubated at 28 to 30°C. Gram-negative, rod-shaped bacteria were consistently isolated from stems with pith necrosis. They were identified as Pseudomonas corrugata using Biolog analysis (carbon source utilization at 37°C), with a similarity index of 1.0. To confirm pathogenicity, experiments were conducted twice in a screenhouse on a total of 24 2-month-old tomato cv. Rutgers plants (50 to 60 cm tall). Bacteria were injected with a syringe into the stems above the point of lateral branching at two different sites, using 0.25 to 0.5 ml of a bacterial suspension (105 CFU/ml). Injection points were sealed after inoculation with a small amount of petroleum jelly. Necrotic lesions surrounding the point of injection were observed 10 days after inoculation. Four weeks after inoculation, plants showed necrotic pith symptoms similar to those observed on field-grown plants, and P. corrugata was recovered from the margins of areas with necrotic pith. Control plants, which were injected with water, remained healthy throughout the experiments. P. corrugata has been reported to cause pith necrosis in tomato plants in California (3), Florida (2), and the northern part of the Baja California peninsula (1). This report confirms the presence of P. corrugata in the Baja California peninsula, and to our knowledge, this is the first report of P. corrugata causing pith necrosis in tomato plants in the southern state of Baja California Sur, Mexico. References: (1) N. B. Carroll et al. N.C. Agric. Res. Serv. Tech. Bull. No. 300, 1992. (2) J. B. Jones et al. Plant Dis. 67:425, 1983. (3) M. Lai et al. Plant Dis. 67:110, 1983.

Pythium aphanidermatum Causing Collar Rot on Papaya in Baja California Sur, Mexico.

Demand from international markets for organically grown papaya (Carica papaya L.) from Baja California Sur is increasing. Occasional rains during the summer of 2000 provided extra moisture to the soil in most papaya farms in the state. Collapsed plants of the Hawaiian type cv. Sunset were observed in 20 commercial orchards in Pescadero, Baja California Sur from July through September 2000. The average disease incidence per orchard was 2%, although there was one orchard with 25% of diseased plants. The initial symptoms were soft, watery lesions at the soil line. As collar rot progressed, foliage wilted. In plants with severe collar rotting, lesions girdled the stem, causing the foliage to be completely wilted and the plants to collapse. Root rot was not observed in plants with collar rot. To isolate the pathogen, 15-cm-long portions of the stem with rot lesions were excised, washed with soap and brush, and rinsed with tap water. Transverse sections of the stem were lightly sprayed with 95% ethanol, and the ethanol was ignited. The superficially burned tissue was removed aseptically, and 1-cm-square sections were cut from the remaining tissue. These sections were plated on potato dextrose agar. The fungus consistently isolated from disease stems grew optimally at 37°C, producing lobate sporangia, antheridia mostly intercalary, and aplerotic oospores characteristic of Pythium aphanidermatum (Edson) Fitzp. (3). Pathogenicity studies were conducted twice in a screened house on a total of 36 4 to 6 month-old Sunset papaya plants 85 to 100 cm tall. Two longitudinal wounds (0.5 cm long and 0.2 cm deep) were made on opposite sides at the base of the stem using sterile razor blades. Pathogen inoculum was obtained from 7-day-old V8 agar cultures. Thirty milliliters of an oospore suspension (200 oospores per ml) and V8 agar plugs containing mycelia and oospores were applied next to the crown of wounded and nonwounded plants. Initial symptoms were observed 3 days after inoculation and were similar to those observed on diseased plants in the field. Wounded, pathogen-inoculated plants were dead 6 days after inoculation. P. aphanidermatum was reisolated from diseased plants. Nonwounded pathogen-inoculated plants, wounded water-inoculated plants, and nonwounded water-inoculated plants remained healthy throughout the experiments. Pathogenicity experiments suggest that field grown papaya plants might be predisposed to infection by P. aphanidermatum due to mechanical damage to the base of the stem caused by abiotic factors such as wind driven sand. P. aphanidermatum has been reported to cause root rot in C. papaya in Tabasco, Mexico (2), and the United States (1). This is the first report of P. aphanidermatum causing collar rot on C. papaya in Baja California Sur. References: (1) Anonymous. 1960. Index of Plant Diseases in the United States. USDA. Handbook. No. 165. Washington, DC. (2) M. I. Saldaña et al. Rev. Mex. Fitopatol. 3:14, 1985. (3) A. J. Van der Plaats-Niterink. Studies Mycol. 21:1, 1981.