Spatiotemporal association between the mite Brevipalpus yothersi and Citrus leprosis virus C in orange orchards.

Citrus leprosis virus C (CiLV-C) is an economically important pathogen and the main causative agent of leprosis disease in citrus orchards. The main vector of this disease, the mite Brevipalpus yothersi, is widely distributed in Mexican orchards on a wide range of citrus species. Despite the importance of both the virus and the mite, field studies recording their occurrence and co-occurrence are practically non-existent. We systematically sampled orange orchards for both CiLV-C and B. yothersi throughout the year. The distribution of the CiLV-C and B. yothersi was evaluated on each sampling occasion and their spatiotemporal associations were determined. Specifically, 100-112 orange trees, distributed in 18 rows (five or six trees per row), were sampled monthly between March 2017 and February 2018 (11 sampling dates). Twenty leaves per tree were sampled on each occasion. The number of mites per tree and the percentage of leaves per tree with disease symptoms were recorded. On each sampling occasion, spatiotemporal associations between mites and disease were determined using the Spatial Analysis by Distance Indices (SADIE) method. CiLV-C and B. yothersi were identified using molecular methods. Throughout the study, the distribution of CiLV-C was aggregated and the distribution of B. yothersi was random. No association was found between the virus and the mite on any of the sampling dates. In total, 173 mites were collected, but only 43 mites were found to be carrying CiLV-C. The reason for this lack of association between the virus and the mite, as well as the impact of our findings on the epidemiology of the disease in orange orchards, are discussed.

Population Growth and Characterization of Plant Injuries of Steneotarsonemus spinki Smiley (Acari: Tarsonemidae) on Rice.

Rice is attacked by Steneotarsonemus spinki Smiley, a mite that has dispersed throughout many countries causing important loss on rice production. Rice plants of the variety Morelos A-92 were infested with S. spinki, and its population growth was estimated along plant development. Further, the morphological and histological injuries associated to the mite attack were characterized. The highest infestation level was obtained 13 weeks after plant infestation, with an average of 58.5 mites per plant, predominantly females. Morphological injuries were categorized from level 0 (no injuries from uninfested plants) to level 3, characterized by the highest injuries represented by blotches on the adaxial epidermis of the leaf sheath and on panicles and grains. Plants ranked within levels 0, 1, and 2 for morphological injury did not exhibit clear histological injuries, while those at level 3 exhibited histological injury characterized by destruction of cells of the adaxial epidermis, disorder, color change, and hypertrophy in the mesophyll cells, as well as color change in the abaxial epidermis. Thus, it presented a significant correlation between morphological injuries and mite density level, which can be further adopted to help the control decision-making process for this mite on rice.

First Report of Leaf Rust of Blueberry Caused by Thekopsora minima in Mexico.

Blueberry (Vaccinium corymbosum L.) is becoming an important crop in the states of Jalisco and Michoacan in Mexico. Leaf rust, a disease causing extensive defoliation on plants with severe infections, was observed in the autumn of 2007 and it has become one of the most significant diseases of blueberry in these states. Symptoms on the upper surfaces of leaves appear as small, yellow spots that later turn necrotic as they enlarge and coalesce and eventually cover large areas of individual leaves. On the undersides of leaves, small flecks surrounded by small water-soaked halos appear, turn yellow, and produce powdery sori that are uredinia with urediniospores. Uredinia were hypophyllous, scattered to gregarious and at times superficially appearing confluent, up to about 300 µm in diameter, dome shaped and peridium hemispherical in cross section, orangish, becoming pulverulent, lacking obviously enlarged, well-differentiated ostiolar cells. Urediniospores were subglobose, obovate, oblong or ellipsoid, 17.6 to 27.2 × 12.8 to 17.6 µm, with hyaline, echinulate walls that are 1.2 to 1.8 µm thick, and with yellow-to-hyaline contents. Telia were not observed. On the basis of uredinial morphology (3,4), the rust was identified as Thekopsora minima P. Syd. & Syd. To distinguish this rust from other rust species causing disease on Vaccinium (2,3), a 1,414-bp region consisting of ITS2 and the 5' end of the 28S was amplified with primers Rust2inv/LR6 from uredinial lesions on infected leaves of V. corymbosum 'Biloxi' and sequenced (BPI 880580; GenBank Accession No. HM439777) (1). Results of a BLAST search of GenBank found 100% (1,414 of 1,414) identity to T. minima (GenBank Accession No. GU355675) from South Africa (3). Pathogenicity tests were completed as follows: (i) during the autumn of 2009, rusted leaves of cvs. Biloxi and Sharpblue were collected from the field; (ii) mature leaves from healthy plants of both blueberry cultivars were surface disinfested with 1% sodium hypochlorite for 2 min and rinsed with sterile distilled water; (iii) fresh urediniospores from rusted leaves were brushed directly onto the undersides of disinfested detached leaves; (iv) to avoid drying, wet cotton balls were placed on the petioles of inoculated leaves that were subsequently placed in resealable plastic bags; and (v) leaves were then incubated in a growth chamber at 22°C with a 12-h photoperiod. For each cultivar, 20 leaves were inoculated and five uninoculated leaves were included as controls and the test was repeated once. Yellow uredinia were observed 13 and 10 days after inoculation in cvs. Biloxi and Sharpblue, respectively. Leaf symptoms and uredinial characters were the same as observed previously in the field. To our knowledge, this is the first report of T. minima in Mexico. This report is significant for growers who need a diagnosis to control the disease and for breeders and plant pathologists who should consider developing more resistant cultivars. References: (1) M. C. Aime. Mycoscience 47:112, 2006. (2) F. L. Caruso and D. C. Ramsdell, eds. Compendium of Blueberry and Cranberry Diseases. The American Phytopathological Society, St. Paul, MN, 1995. (3) L. Mostert et al. Plant Dis. 94:478, 2010. (4) P. Sydow and H. Sydow. Monographia Uredinearum. Vol. III. Fratres Borntraeger, Leipzig, Germany, 1915.