Search

Phytophthora cactorum is the most common causal agent of Phytophthora crown rot and leather rot of strawberry, but P. nicotianae is also responsible for the disease in Florida. Studies of P. nicotianae populations have suggested that different groups of genotypes are associated with different hosts; however, it is not yet clear how many lineages exist globally and how they are related to different production systems. The aim of this study was to determine the genetic relationships of P. nicotianae isolates from Florida strawberry with genotypes reported from other hosts, quantify the genetic variation on strawberry, and test for an association with nursery source. A total of 49 isolates of P. nicotianae were collected from strawberry plants originating from multiple nursery sources during six seasons of commercial fruit production in Florida. Microsatellite genotyping identified 28 multilocus genotypes on strawberry that were distinct among 208 isolates originating from various hosts and locations. Based on STRUCTURE analysis, two genetic groups were identified: one consisting of isolates from strawberry, and the other comprising samples from different hosts. Multilocus genotypes were shared among nursery sources, and populations defined by nursery were not differentiated. Both mating types were found among the isolates from North Carolina- and California-origin plants and in most strawberry seasons; however, a predominance of A1 was observed, and regular sexual reproduction was not supported by the data. This study reveals a unique genetic population of P. nicotianae associated with strawberry and emphasizes the vital role of nursery monitoring in mitigating disease spread. [ABSTRACT FROM AUTHOR]

This report investigates population structure and genetic variability of Phytophthora spp. isolated from botanically diverse plants in Florida. Internal transcribed spacer-based molecular phylogenetic analyses indicate that Phytophthora isolates recovered from ornamental plants in Florida represent a genetically diverse population and that a majority of the isolates belong to Phytophthora nicotianae (73.2%), P. palmivora (18.7%), P. tropicalis (4.9%), P. katsurae (2.4%), and P. cinnamomi (0.8%). Mating type analyses revealed that most isolates were heterothallic, consisting of both mating type A1 (25.2%) and mating type A2 (39.0%), and suggesting that they could outcross. Fungicide sensitivity assays determined that several isolates were moderate to completely insensitive to mefenoxam. In addition, several isolates were also moderately insensitive to additional fungicides with different modes of action. However, correlation analyses did not reveal occurrence of fungicide cross-resistance. These studies suggest that a genetically diverse Phytophthora population infects ornamental crops and the occurrence of mefenoxam-insensitive Phytophthora populations raises concerns about disease management in ornamentals. Mitigating fungicide resistance will require prudent management strategies, including tank mixes and rotation of chemicals with different modes of actions. [ABSTRACT FROM AUTHOR]

Aims: Phytophthora root rot of citrus in Florida is caused by Phytophthora nicotianae and P. palmivora. In a field experiment, pre-inoculation of citrus fibrous roots before planting with a naturally occurring isolate of P. nicotianae (Pn117) reduced root rot caused by virulent P. nicotianae in an infested citrus orchard. On potted citrus rootstock seedlings, Pn117 caused less disease, but was not impaired in ability to infect and colonize roots compared to virulent Phytopththora spp. In a competition experiment, pre-inoculation of citrus rootstock seedlings with Pn117 followed by inoculation with virulent isolates of P. nicotianae (Pn198) and P. palmivora (Pp99) resulted in an increase in recovery of Pn117 and a decrease in recovery of virulent isolates compared to the co-inoculation treatment. Methods: To investigate whether induction of systemic acquired resistance (SAR) by Pn117 is a mechanism for protection of roots against the virulent isolates, the chitinase gene CsPR4 was cloned from a differentially expressed cDNA library from Valencia sweet orange ( Citrus sinensis). Sour orange ( C. aurantium) seedling roots were exposed to zoospores of Pn117, Pn198, or Pp99 for 2, 6, 24, and 48 h. Seedling roots were also treated with solutions of 100 μM salicylic acid (SA), benzothiadiazole (BTH), and jasmonic acid (JA). A split root system experiment was conducted to determine if CsPR4 is systemically induced by infection. One side of the root system was inoculated with zoospores of Pn117, Pn198, or Pp99 and the other side was placed in water. Results: CsPR4 transcript accumulated at 2 h after inoculation with Pn117, whereas accumulation in response to Pp99 and Pn198 was delayed until 6 and 24 h, respectively. Systemic resistance-inducing SA, JA, or BTH, upregulated CsPR4 implicating this gene as an inducible component of defense pathways in citrus. In the split root assay, infection by Pn117 failed to induce CsPR4 in distal roots. Conclusion: Lack of response of CsPR4 in distal roots to hypovirulent Pn117 did not confirm a role for SAR in protection of citrus roots, but reinforces field and greenhouse findings that pre-inoculation with the hypovirulent isolate Pn117 protects roots from virulent isolates by competitive exclusion through pre-emptive infection and reproduction on roots. [ABSTRACT FROM AUTHOR]

Phytophthora crown rot, caused mainly by Phytophthora cactorum but also by P. nicotianae , reported in 2018, is an important disease in the Florida strawberry annual production system. Mefenoxam is the most effective and widely used fungicide to manage this disease. However, because of pathogen resistance, alternatives to chemical control are needed. Phytophthora spp. were rarely recovered during the summer from soil of commercial farms where the disease was observed during the season. In a more detailed survey on research plots, neither of the two species was recovered 1 month after the crop was terminated and water was shut off. Therefore, Phytophthora spp. does not seem to survive in the soil over summer in Florida. In a field trial, asymptomatic nursery transplants harboring quiescent infections were confirmed as the major source of inoculum for these pathogens in Florida. Heat treatment of P. cactorum zoospores at 44°C for as little as 5 min was effective in inhibiting germination and colony formation; however, oospore germination was not inhibited by any of the tested temperatures in vitro. In the field, thermotherapy treatment of inoculated plants was shown to have great potential to serve as a nonchemical approach for managing Phytophthora crown rot in production fields and reducing mefenoxam-resistant populations in nursery transplants.