Alternaria diseases of citrus - Novel pathosystems

Citrus is affected by four diseases caused by Alternaria spp. Brown spot of tangerines, leaf spot of rough lemon, postharvest black rot of fruit occur widely in citrus areas of the world and are caused by different pathotypes of A. alternata. Mancha foliar occurs only on Mexican lime in western Mexico and is caused by A. limicola. Tangerine and rough lemon pathotypes produce host-specific toxins that affect membranes and respiration, respectively. Black rot is always associated with wounds and is caused by most citrus-associated isolates of A. alternata that produce endopolygalacturonase. Alternaria brown spot is a serious disease of susceptible tangerines and their hybrids in semi-arid Mediterranean climates as well as in more humid areas. Conidia, produced on lesions on mature and senescent leaves and stems under humid conditions, are dispersed by wind, and infect all juvenile tissues of susceptible cultivars when temperature and leaf wetness conditions are favorable. Commercially acceptable cultivars resistant to brown spot are being developed. Disease severity can be reduced by planting disease-free nursery stock on wider spacings, pruning tree skirts, and reducing irrigation and nitrogen fertilization. However, fungicides such as dithiocarbamates, triazoles, strobilurins, iprodione, or copper fungicides are used in most areas for disease control. A disease-forecasting model, the Alter-Rater, has been developed in Florida to assist in timing fungicide sprays

Genetic diversity of the myrtle rust pathogen (Austropuccinia psidii) in the Americas and Hawaii : global implications for invasive threat assessments

Since the myrtle rust pathogen (Austropuccinia psidii) was first reported (as Puccinia psidii) in Brazil on guava (Psidium guajava) in 1884, it has been found infecting diverse myrtaceous species. Because A. psidii has recently spread rapidly worldwide with an extensive host range, genetic and genotypic diversities were evaluated within and among A. psidii populations in its putative native range and other areas of myrtle rust emergence in the Americas and Hawaii. Microsatellite markers revealed several unique multilocus genotypes (MLGs), which grouped isolates into nine distinct genetic clusters [C1–C9 comprising C1: from diverse hosts from Costa Rica, Jamaica, Mexico, Puerto Rico, and USA‐Hawaii, and USA‐California; C2: from eucalypts (Eucalyptus spp.) in Brazil/Uruguay and rose apple (Syzygium jambos) in Brazil; C3: from eucalypts in Brazil; C4: from diverse hosts in USA‐Florida; C5: from Java plum (Syzygium cumini) in Brazil; C6: from guava and Brazilian guava (Psidium guineense) in Brazil; C7: from pitanga (Eugenia uniflora) in Brazil; C8: from allspice (Pimenta dioica) in Jamaica and sweet flower (Myrrhinium atropurpureum) in Uruguay; C9: from jabuticaba (Myrciaria cauliflora) in Brazil]. The C1 cluster, which included a single MLG infecting diverse host in many geographic regions, and the closely related C4 cluster are considered as a “Pandemic biotype,” associated with myrtle rust emergence in Central America, the Caribbean, USA‐Florida, USA‐Hawaii, Australia, China‐Hainan, New Caledonia, Indonesia and Colombia. Based on 19 bioclimatic variables and documented occurrences of A. psidii contrasted with reduced sets of specific genetic clusters (subnetworks, considered as biotypes), maximum entropy bioclimatic modelling was used to predict geographic locations with suitable climate for A. psidii which are at risk from invasion. The genetic diversity of A. psidii throughout the Americas and Hawaii demonstrates the importance of recognizing biotypes when assessing the invasive threats posed by A. psidii around the globe.USDA-Forest Service, RMRS-Forest and Woodlands Ecosystem Program, Western Wildlands Environmental Threat Assessment Center, Special Technology Development Program, State and Private Forestry, Forest Health Protection-Region 5; Conselho Nactional de Desenvolvimento Científico e Tecnológico, Brasil (CNPq); Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG); Research Joint Venture Agreements RMRS 15-JV-11221633-160 (Jane Stewart, Colorado State University) and RMRS 14-JV-11221633-117 (Western Forest Conservation Association).http://wileyonlinelibrary.com/journal/efp2019-02-01hj2018Forestry and Agricultural Biotechnology Institute (FABI)Plant Production and Soil Scienc

Temperature, leaf wetness, and isolate effects on infection of Minneola tangelo leaves by Alternaria sp.

Alternaria brown spot causes necrotic lesions on immature leaves, twigs, and fruit of tangerines and their hybrids, reducing yield and fruit quality. The effect of temperature, leaf wetness, and isolate was evaluated in an in vitro system using immature detached leaves of Minneola tangelo. Infection was greatest at 27°C, decreased gradually as the temperature declined to 24, 20, and 17°C, and dropped sharply at 32°C. Levels of infection were low at 4 and 8 h of leaf wetness and continued to increase with longer wetting periods up to 36 h. A polynomial equation was developed that provided a good fit for the data (adjusted R2 = 0.93). Isolates differed in aggressiveness, but there was no significant difference among isolates in their response to temperature and leaf wetness duration

Population genetic structure and host specificity of Alternaria spp. causing brown spot of Minneola tangelo and rough lemon in Florida

Alternaria spp. were sampled from two rough lemon (RL) and two Minneola tangelo (MIN) groves in a limited geographic area in central Florida to test for host-specialized forms of the pathogen. Isolates of Alternaria spp. were scored for variation at 16 putative random amplified polymorphic DNA (RAPD) loci and for pathogenicity on both hosts. Subpopulations on each host were differentiated genetically and pathogenically, which was consistent with the hypothesis of host specialization. Highly significant genetic differentiation was detected among all four subpopulations (Nei's coefficient of gene differentiation [G(ST)] = 0.292, P = 0.000); most of the differentiation occurred between hosts (G(ST) = 0.278, P = 0.000). Phenograms of qualitative similarities among isolates within subpopulations revealed two or three distinct clusters of isolates within each subpopulation. The majority of isolates sampled from RL were pathogenic on RL and not on MIN, although a few RL isolates were able to induce disease on MIN, and 44% were nonpathogenic on either host. In contrast, isolates from MIN were pathogenic only on MIN, never on RL, and only 3% of the isolates were nonpathogenic. Overall, three genetically distinct clusters of isolates were detected on both hosts. One of the clusters (cluster A) sampled from RL was pathogenic on RL and not on MIN and consisted almost entirely of one RAPD genotype. This cluster also contained two isolates that were 93% similar to the majority genotype but were pathogenic on MIN and not RL. In isolates from MIN, two distinct clusters of isolates were found in one subpopulation (clusters B and C), and three distinct clusters were found in another subpopulation (clusters A, B, and C). Clusters A and B were found on both hosts, while cluster C was limited to MIN. Populations of Alternaria spp. sampled from RL and MIN showed a high degree of host specificity; however, the specificity obscured a high level of genetic variation within subpopulations