First Report of Fusarium solani associated with Twig Canker of English Walnut (Juglans regia L.) in Xinjiang, China.

English walnut (Juglans regia L.) is a perennial deciduous fruit tree, and an economically important hardwood tree species cultivated worldwide. As one of the important economic crops, English walnut is also widely cultivated in Xinjiang. In September 2019, twig canker symptoms were observed on English walnut in several orchards, with approximately 15% to 40% disease incidence in southern Xinjiang region (79º95'E, 40º37'N). The branch lesions were long oval, concave, and black to brown. Leaves of the affected branches turned yellow and the branches eventually died. Infected twigs were collected from an infected tree in an orchard. Symptomatic tissue from the margins of cankers was surface disinfested with 75% ethanol for 60 s, rinsed 3 times with sterile water, and then incubated on potato dextrose agar medium (PDA) at 25 ℃ under a 12-hr photoperiod in Light incubator for 7 days. Seven fungal isolates showing similar morphology were obtained from the symptomatic tissue. All the fungal cultures had a pink-white color with loose, cottony mycelium, and the underside of the colonies were light brown. Macroconidia were slightly curved, with one to six septa, both ends were slightly sharp, and they measured 22.8 to 38.5 × 3.5 to 6.7 µm (27.4 ± 0.6 × 4.2 ± 0.3 µm, n=50). Microconidia were oval, hyaline, zero to one septa, measuring 4.5 to 9.6 × 1.8 to 2.3 µm (6.8 ± 0.3 × 2.1 ± 0.1 µm, n=50). According to the morphological characteristics, the seven isolates were identified as a member of the Fusarium solani species complex (Summerell et al. 2003). Genomic DNA was extracted from the representative isolate HSANTUAN2019-1, and the internal transcribed spacer (ITS) region, the translation elongation factor 1-alpha (TEF) were amplified with the primer pairs ITS1/ITS4 (White et al. 1990) and EF1-F/EF2-R (O' Donnell et al. 2010), respectively. The sequences submitted to GenBank (accession nos. OP271472 for ITS, OP293104 for TEF) showed high similarity with the reference sequences of F. solani (ITS, OL691083 [100%]; TEF, HE647960 [99.86%]). Pathogenicity of the seven isolates were assessed on 1-year-old branches of English walnut in the field. Healthy branches (40) were wounded with a sterilized hole punch, and then inoculated with isodiametric mycelial PDA plugs (5 branches per fungal isolate). Five branches were inoculated with sterile PDA plugs as a negative control. The inoculations were performed three times. All treatments were wrapped with fresh film for 3 days. Dark brown necrotic lesions were observed on all inoculated branches 22 days post-inoculation. The controls had no symptoms. The pathogen was reisolated from all the inoculated branches, fulfilling Koch's postulates. To our knowledge, this is the first report of F. solani causing twig canker on English walnut in Xinjiang, China. Twig canker disease often cause a large number of branches to dry out and die. If the disease control and prevention is neglected, the productivity of the English walnut will be seriously affected in the cultivation area. Our finding will provide valuable information for prevention and management of twig canker on English walnut.

Accuracy Analysis of International Reference Ionosphere 2016 and NeQuick2 in the Antarctic.

Global navigation satellite system (GNSS) can provide dual-frequency observation data, which can be used to effectively calculate total electron content (TEC). Numerical studies have utilized GNSS-derived TEC to evaluate the accuracy of ionospheric empirical models, such as the International Reference Ionosphere model (IRI) and the NeQuick model. However, most studies have evaluated vertical TEC rather than slant TEC (STEC), which resulted in the introduction of projection error. Furthermore, since there are few GNSS observation stations available in the Antarctic region and most are concentrated in the Antarctic continent edge, it is difficult to evaluate modeling accuracy within the entire Antarctic range. Considering these problems, in this study, GNSS STEC was calculated using dual-frequency observation data from stations that almost covered the Antarctic continent. By comparison with GNSS STEC, the accuracy of IRI-2016 and NeQuick2 at different latitudes and different solar radiation was evaluated during 2016-2017. The numerical results showed the following. (1) Both IRI-2016 and NeQuick2 underestimated the STEC. Since IRI-2016 utilizes new models to represent the F2-peak height (hmF2) directly, the IRI-2016 STEC is closer to GNSS STEC than NeQuick2. This conclusion was also confirmed by the Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) occultation data. (2) The differences in STEC of the two models are both normally distributed, and the NeQuick2 STEC is systematically biased as solar radiation increases. (3) The root mean square error (RMSE) of the IRI-2016 STEC is smaller than that of the NeQuick2 model, and the RMSE of the two modeling STEC increases with solar radiation intensity. Since IRI-2016 relies on new hmF2 models, it is more stable than NeQuick2.

Analysis of Ionospheric Disturbances Caused by the 2018 Bering Sea Meteor Explosion Based on GPS Observations.

The Bering Sea meteor explosion that occurred on 18 December 2018 provides a good opportunity to study the ionospheric disturbances caused by meteor explosions. Total electron content (TEC) is the core parameter of ionospheric analysis. TEC and its changes can be accurately estimated based on the Global Positioning System (GPS). TID is detected in time and frequency domain based on power spectrum and Butterworth filtering method. By analyzing the waveform, period, wavelength, propagation speed and space-time distribution of TID, the location of the TID source is determined, and the process of TID formation and propagation is understood. The TID caused by meteor explosions has significant anisotropy characteristic. Two types of TID were found. For the first type, the average horizontal propagation velocity is 250.22 ± 5.98 m/s, the wavelength is ~135-240 km, the average period is about 12 min, and the propagation distance is less than 1400 km. About 8 min after the meteor explosion, the first type of TID source formed and propagated radially at the velocity of 250.22 ± 5.98 m/s. For the second type, the propagation velocity is ~434.02 m/s. According to the waveform, period, wavelength and propagation velocity of the TID, it is diagnosed to be the midscale traveling ionospheric disturbances (MSTID). Based on the characteristics of TID, we infer that the TID is excited by the gravity waves generated by the meteor explosion, which is in accordance with the propagation law of gravity waves in the ionosphere. And it is estimated that the average velocity of the up-going gravity waves is about 464.58 m/s. A simple model was established to explain the formation and the propagation of this TID, and to verify the characteristics of the TID propagation caused by nuclear explosion, earthquake, tsunami, and Chelyabinsk meteorite blast. It is estimated that the position of the TID source is consistent with the meteor explosion point, which further indicates that the TID is caused by the meteor explosion and propagates radially.

Real-Time Global Ionospheric Map and Its Application in Single-Frequency Positioning.

The prevalence of real-time, low-cost, single-frequency, decimeter-level positioning has increased with the development of global navigation satellite systems (GNSSs). Ionospheric delay accounts for most errors in real-time single-frequency GNSS positioning. To eliminate ionospheric interference in real-time single-frequency precise point positioning (RT-SF-PPP), global ionospheric vertical total electron content (VTEC) product is designed in the next stage of the International GNSS Service (IGS) real-time service (RTS). In this study, real-time generation of a global ionospheric map (GIM) based on IGS RTS is proposed and assessed. There are three crucial steps in the process of generating a real-time global ionospheric map (RTGIM): estimating station differential code bias (DCB) using the precise point positioning (PPP) method, deriving slant total electron content (STEC) from PPP with raw observations, and modeling global vertical total electron content (VTEC). Experiments were carried out to validate the algorithm's effectiveness. First, one month's data from 16 globally distributed IGS stations were used to validate the performance of DCB estimation with the PPP method. Second, 30 IGS stations were used to verify the accuracy of static PPP with raw observations. Third, the modeling of residuals was assessed in high and quiet ionospheric activity periods. Afterwards, the quality of RTGIM products was assessed from two aspects: (1) comparison with the Center for Orbit Determination in Europe (CODE) global ionospheric map (GIM) products and (2) determination of the performance of RT-SF-PPP with the RTGIM. Experimental results show that DCB estimation using the PPP method can realize an average accuracy of 0.2 ns; static PPP with raw observations can achieve an accuracy of 0.7, 1.2, and 2.1 cm in the north, east, and up components, respectively. The average standard deviations (STDs) of the model residuals are 2.07 and 2.17 TEC units (TECU) for moderate and high ionospheric activity periods. Moreover, the average root-mean-square (RMS) error of RTGIM products is 2.4 TECU for the one-month moderate ionospheric period. Nevertheless, for the high ionospheric period, the RMS is greater than the RMS in the moderate period. A sub-meter-level horizontal accuracy and meter-level vertical accuracy can be achieved when the RTGIM is employed in RT-SF-PPP.

Establishing a method of short-term rainfall forecasting based on GNSS-derived PWV and its application.

ABSTARCT: Global Navigation Satellite System (GNSS) can effectively retrieve precipitable water vapor (PWV) with high precision and high-temporal resolution. GNSS-derived PWV can be used to reflect water vapor variation in the process of strong convection weather. By studying the relationship between time-varying PWV and rainfall, it can be found that PWV contents increase sharply before raining. Therefore, a short-term rainfall forecasting method is proposed based on GNSS-derived PWV. Then the method is validated using hourly GNSS-PWV data from Zhejiang Continuously Operating Reference Station (CORS) network of the period 1 September 2014 to 31 August 2015 and its corresponding hourly rainfall information. The results show that the forecasted correct rate can reach about 80%, while the false alarm rate is about 66%. Compared with results of the previous studies, the correct rate is improved by about 7%, and the false alarm rate is comparable. The method is also applied to other three actual rainfall events of different regions, different durations, and different types. The results show that the method has good applicability and high accuracy, which can be used for rainfall forecasting, and in the future study, it can be assimilated with traditional weather forecasting techniques to improve the forecasted accuracy.