First Report of Lasiodiplodia pseudotheobromae causing Postharvest Decay of Strawberries in Florida

Postharvest decay of strawberry (Fragaria × ananassa Duch.) is a major factor causing fruit losses. Strawberries were obtained from various harvests at cooling facilities located in Dover and Plant City, FL during the 2018-19 and 2019-20 seasons. After the fruits were incubated at 22ºC for up to 5 days (d) to promote disease development, Lasiodiplodia decay was observed at up to 3% from some harvests, exhibiting gray mycelia on small lesions that gradually covered the whole fruit. The fungus was isolated onto potato dextrose agar (PDA). Five isolates (SBD18-14, SBD18-277, SBD18-279, SBD19-02 and SBD19-57) were characterized. Fungal mycelia were initially grayish white and then gradually changed to gray to dark gray on PDA at 25oC, and later produced black pigments (Fig. S1). Pycnidia were observed from inoculated strawberries at 14 d. Isolates shared similar conidia morphology: aseptate, hyaline, ellipsoid to ovoid, measuring L × W: 24.0-34.0 (28.3) × 13.0-16.0 (14.3) μm (n =100). Mature conidia were brown, one septate, measuring L × W: 25.0-33.0 (28.8) × 13.0-16.0 (14.5) μm (n =100). The isolates were identified as Lasiodiplodia spp. morphologically (Alves et al. 2008). DNA was extracted from fungal mycelia using an OmniPrep DNA extraction kit, and PCR amplification of ITS and EF1-α genes was performed following the conditions described by White et al. (1990) with some modifications using primers ITS1F-F/ITS4-R (Gardes and Bruns, 1993; White et al., 1990) and EF1-668-F/EF1-1251-R (Alves et al., 2008), respectively. The BLASTn in GenBank showed that the sequences obtained had 99.61 to 100% homology with those of ITS (EF622077) and EF1-α (EF622057) from L. pseudotheobromae CBS116459 (an ex-type strain) (Alves et al., 2008). Sequences of the isolates have been deposited in GenBank with accessions OP326017 to OP326021 for ITS, and OP356202 to OP356206 for EF1-α. Phylogenetic analysis showed that these isolates clustered in the same clade (bootstrap value at 64) with L. pseudotheobromae (Fig. S2). Two fungal inoculum types (mycelia and conidia), two fruit inoculation methods (injury and non-injury) and five fungal isolates were used for pathogenicity tests. Fungal mycelia (2-day-old) on PDA plug (5 mm) or 10 µL of conidial suspension (106 spores/mL) was placed onto each injury (1 x 1 mm in size) or a non-injury area on the surfaces of five strawberry fruits (cv. Florida Brilliance). PDA plug alone or water drops placed on injury or non-injury areas on fruits served as respective controls. Inoculated and control fruits were incubated in a covered plastic container with 100% RH at 22ºC. The experiment was repeated twice. Decay initially appeared as soft and lightly discolored tissue at inoculation areas 2 d post-inoculation (dpi) that extended quickly thereafter. Brown to dark lesions on both injury- and non-injury fruits inoculated with conidia or mycelia were observed at 3 dpi. Decay and gray mycelia gradually developed over the whole fruit at 6 dpi, and pycnidia were observed after 14 dpi (Fig. S1). Disease incidence of 100% was observed on all tests. Control fruits did not develop decay. The results indicate that these isolates are pathogenic to strawberries and infect fruit via both non-injured and injured fruit surfaces. The inoculated fungal isolates were re-isolated, thus, fulfilling Koch's postulates. L. theobromae, Neofusicoccum parvum/N. ribis species complex causing strawberry fruit rot in Florida fields was reported (Oliveira et al., 2019), but not L. pseudotheobromae. To our knowledge, this is the first report of postharvest decay caused by L. pseudotheobromae A.J.L. Phillips, A. Alves & Crous on strawberries in Florida and in the USA, and it should be considered in strawberry disease management.

Draft Genome Sequence Resource for the Orange Rust Pathogen of Sugarcane Puccinia kuehnii

Puccinia kuehnii is an obligate biotrophic fungal pathogen that causes orange rust of sugarcane, which is prevalent in many countries around the globe. In the United States, orange rust was first detected in sugarcane in Florida in 2007 and poses a persistent and economically damaging threat to the sugarcane industry in this region. Here, we generated the first genome assemblies for two isolates of P. kuehnii (1040 and 2143) collected in Florida in 2017 from two sugarcane cultivars, CL85-1040 and CP89-2143, respectively. These two rust genome resources will be of immense value for future genomic studies, particularly further exploration of the predicted secretomes that may help define key pathogenicity determinants for this economically important pathogen.

Sub-cycle dynamics in relativistic nanoplasma acceleration

The interaction of light with nanometer-sized solids provides the means of focusing optical radiation to sub-wavelength spatial scales with associated electric field enhancements offering new opportunities for multifaceted applications. We utilize collective effects in nanoplasmas with sub-two-cycle light pulses of extreme intensity to extend the waveform-dependent electron acceleration regime into the relativistic realm, by using 106 times higher intensity than previous works to date. Through irradiation of nanometric tungsten needles, we obtain multi-MeV energy electron bunches, whose energy and direction can be steered by the combined effect of the induced near-field and the laser field. We identified a two-step mechanism for the electron acceleration: (i) ejection within a sub-half-optical-cycle into the near-field from the target at >TVm-1 acceleration fields, and (ii) subsequent acceleration in vacuum by the intense laser field. Our observations raise the prospect of isolating and controlling relativistic attosecond electron bunches, and pave the way for next generation electron and photon sources.

Next Generation Driver for Attosecond and Laser-plasma Physics

The observation and manipulation of electron dynamics in matter call for attosecond light pulses, routinely available from high-order harmonic generation driven by few-femtosecond lasers. However, the energy limitation of these lasers supports only weak sources and correspondingly linear attosecond studies. Here we report on an optical parametric synthesizer designed for nonlinear attosecond optics and relativistic laser-plasma physics. This synthesizer uniquely combines ultra-relativistic focused intensities of about 10²⁰ W/cm² with a pulse duration of sub-two carrier-wave cycles. The coherent combination of two sequentially amplified and complementary spectral ranges yields sub-5-fs pulses with multi-TW peak power. The application of this source allows the generation of a broad spectral continuum at 100-eV photon energy in gases as well as high-order harmonics in relativistic plasmas. Unprecedented spatio-temporal confinement of light now permits the investigation of electric-field-driven electron phenomena in the relativistic regime and ultimately the rise of next-generation intense isolated attosecond sources.

Collective Deceleration of Laser-Driven Electron Bunches

Few-fs electron bunches from laser wakefield acceleration (LWFA) can efficiently drive plasma wakefields (PWFs), as shown by their propagation through underdense plasma in two experiments. A strong and density-insensitive deceleration of the bunches has been observed in 2 mm of 10^{18}  cm^{-3} density plasma with 5.1  GV/m average gradient, which is attributed to a self-driven PWF. This observation implies that the physics of PWFs, usually relying on large-scale rf accelerators as drivers, can be studied by tabletop LWFA electron sources.