Interplay of Cucurbit Yellow Stunting Disorder Virus With Cucurbit Chlorotic Yellows Virus and Transmission Dynamics by Bemisia tabaci MED

A Multiplex RT-qPCR Assay for Simultaneous Detection of Cucurbit Viruses from Individual Whitefly and Plant Samples

Whiteflies (Bemisia tabaci) are a significant pest of cucurbits and vector many viruses, leading to substantial economic losses. Modern diagnostic tools offer the potential for early detection of viruses in the whiteflies before crop production. One such tool is the multiplex reverse transcriptase quantitative PCR (RT-qPCR) probe-based technique, which can detect multiple targets in a single reaction and simultaneously quantify the levels of each target, with a detection limit of 100 copies per target. In this study, a multiplex RT-qPCR-based detection system capable of identifying one DNA virus and three RNA viruses in whiteflies-cucurbit leaf crumple virus (CuLCrV), cucurbit chlorotic yellows virus (CCYV), cucurbit yellow stunting disorder virus (CYSDV), and squash vein yellowing virus (SqVYV)-was developed. To ensure the reliability of the assay, an internal gene control as the fifth target to monitor false-negative results was incorporated. This newly developed molecular diagnostic tool possesses several advantages. It can detect up to five desired targets from a single whitefly RNA sample, even at concentrations as low as 1 ng/μl. To evaluate its sensitivity, we conducted experiments using serially diluted cloned plasmids and in vitro transcribed RNA transcripts of the target viruses. We also assessed the specificity of the assay by including aphid-transmitted viruses and other viruses known to infect cucurbits. The diagnostic method successfully detected all five targets simultaneously and allowed for the quantification of up to 100 copies using a mixture of healthy RNA and in vitro transcribed RNA. Our aim with this study was to develop a highly specific and sensitive one-step multiplex RT-qPCR system for the simultaneous detection of viruses transmitted by whiteflies in cucurbits. This system offers significant advantages for early detection, enabling prompt control measures to mitigate the further spread of viral infections and reduce yield losses. Additionally, we demonstrated the ability to simultaneously detect mixed viruses (CCYV, CYSDV, CuLCrV, and SqVYV) in individual whiteflies and quantify the number of viral copies carried by each whitefly. The multiplex RT-qPCR assay outperforms currently available techniques for detecting many samples at a given time and can be effectively utilized for early monitoring of plant viruses in individual whiteflies and symptomless plants.

Whitefly-Transmitted Viruses of Cucurbits in the Southern United States

Cucurbits are economically important crops that are widely cultivated in many parts of the world, including the southern US. In recent years, higher temperatures have favored the rapid build-up of whiteflies in the fall-grown cucurbits in this region. As a result, whitefly-transmitted viruses (WTVs) have severely impacted the marketable yield of cucurbits. In this review, we discuss three major groups of WTVs negatively impacting cucurbit cultivation in the southern US, including begomoviruses, criniviruses, and ipomoviruses. Here, we discuss the available information on the biology, epidemiology and advances made toward detecting and managing these viruses, including sources of resistance and cultural practices.

Small RNA Profiling of Cucurbit Yellow Stunting Disorder Virus from Susceptible and Tolerant Squash (Cucurbita pepo) Lines

RNA silencing is a crucial mechanism of the antiviral immunity system in plants. Small RNAs guide Argonaut proteins to target viral RNA or DNA, preventing virus accumulation. Small RNA profiles in Cucurbita pepo line PI 420328 with tolerance to cucurbit yellow stunting disorder virus (CYSDV) were compared with those in Gold Star, a susceptible cultivar. The lower CYSDV symptom severity in PI 420328 correlated with lower virus titers and fewer sRNAs derived from CYSDV (vsRNA) compared to Gold Star. Elevated levels of 21- and 22-nucleotide (nt) size class vsRNAs were observed in PI 420328, indicating more robust and efficient RNA silencing in PI 420328. The distribution of vsRNA hotspots along the CYSDV genome was similar in both PI 420328 and Gold Star. However, the 3’ UTRs, CPm, and p26 were targeted at a higher frequency in PI 420328.

A New Perspective on the Co-Transmission of Plant Pathogens by Hemipterans

Co-infection of plants by pathogens is common in nature, and the interaction of the pathogens can affect the infection outcome. There are diverse ways in which viruses and bacteria are transmitted from infected to healthy plants, but insects are common vectors. The present review aims to highlight key findings of studies evaluating the co-transmission of plant pathogens by insects and identify challenges encountered in these studies. In this review, we evaluated whether similar pathogens might compete during co-transmission; whether the changes in the pathogen titer in the host, in particular associated with the co-infection, could influence its transmission; and finally, we discussed the pros and cons of the different approaches used to study co-transmission. At the end of the review, we highlighted areas of study that need to be addressed. This review shows that despite the recent development of techniques and methods to study the interactions between pathogens and their insect vectors, there are still gaps in the knowledge of pathogen transmission. Additional laboratory and field studies using different pathosystems will help elucidate the role of host co-infection and pathogen co-transmission in the ecology and evolution of infectious diseases.

Infection Dynamics of Potato Virus Y Isolate Combinations in Three Potato Cultivars

The United States potato industry has recently experienced a strain shift; recombinant potato virus Y (PVY) strains (e.g., PVY^NTN) have emerged as the predominant strains over the long dominant ordinary strain (PVY^O), yet both are often found as single infections within the same field and as mixed infections within individual plants. To understand mixed infection dynamics in potato plants and in daughter tubers, three potato varieties varying for PVY resistance, 'Red Maria', 'CalWhite', and 'Pike', were mechanically inoculated either at the pre- or postflowering stage with all possible heterologous isolate combinations of two PVY^O and two PVY^NTN isolates. Virus titer was determined from leaves collected at different positions on the plant at different times, and tuber-borne infection was determined for two successive generations. PVY^NTN accumulated to higher levels than PVY^O at nearly all sampling time points in 'Pike' potato. However, both virus strains accumulated to similar amounts in 'Red Maria' and 'CalWhite' potato early in the infection when inoculated preflowering; however, PVY^NTN dominated at later stages and in plants inoculated postflowering. Regardless of inoculation time, both virus strains were transmitted to daughter plants raised from the tubers for most isolate combinations. The relative titer of PVY^NTN and PVY^O isolates at the later stages of mother plant development was indicative of what was found in the daughter plants. Although virus titer differed among cultivars depending on their genetics and virus isolates, it did not change the strain outcome in tuber-borne infection in subsequent generations. Differential virus accumulation in these cultivars suggests isolate-specific resistance to PVY accumulation.

Wild Radish (Raphanus raphanistrum L.) Is a Potential Reservoir Host of Cucurbit Chlorotic Yellows Virus

Cucurbit chlorotic yellows virus (CCYV) belongs to the genus Crinivirus and is part of a complex of whitefly-transmitted viruses that cause yellowing disease in cucurbits. In the southeastern USA, heavy incidences of CCYV have been observed on all cucurbits grown in the fall. CCYV was detected from wild radish (Raphanus raphanistrum L.), a common weed that grows in the southeastern USA by high-throughput sequencing as well as RT-PCR. CCYV sequence from wild radish was 99.90% and 99.95%, identical to RNA 1 and RNA 2 of cucurbit isolates of CCYV from the region. Transmission assays using whiteflies demonstrated that wild radish is a good host for CCYV. Whiteflies were also able to acquire CCYV from wild radish and transmit the virus to cucurbit hosts, which developed typical symptoms associated with CCYV. Using quantitative PCR, the titer of CCYV in wild radish was also estimated to be on par with that of cucurbit hosts of the virus. Whitefly bioassays revealed that wild radish is an acceptable feeding and reproductive host plant. These results indicate that wild radish could serve as a reservoir host for CCYV in the USA and other parts of the world where similar conditions exist.

High Throughput Sequencing-Aided Survey Reveals Widespread Mixed Infections of Whitefly-Transmitted Viruses in Cucurbits in Georgia, USA

Viruses transmitted by the sweet potato whitefly (Bemisia tabaci) have been detrimental to the sustainable production of cucurbits in the southeastern USA. Surveys were conducted in the fall of 2019 and 2020 in Georgia, a major cucurbit-producing state of the USA, to identify the viruses infecting cucurbits and their distribution. Symptomatic samples were collected and small RNA libraries were prepared and sequenced from three cantaloupes, four cucumbers, and two yellow squash samples. An analysis of the sequences revealed the presence of the criniviruses cucurbit chlorotic yellows virus (CCYV), cucurbit yellow stunting disorder virus (CYSDV), and the begomovirus cucurbit leaf crumple virus (CuLCrV). CuLCrV was detected in 76%, CCYV in 60%, and CYSDV in 43% of the total samples (n = 820) tested. The level of mixed infections was high in all the cucurbits, with most plants tested being infected with at least two of these viruses. Near-complete genome sequences of two criniviruses, CCYV and CYSDV, were assembled from the small RNA sequences. An analysis of the coding regions showed low genetic variability among isolates from different hosts. In phylogenetic analysis, the CCYV isolates from Georgia clustered with Asian isolates, while CYSDV isolates clustered with European and USA isolates. This work enhances our understanding of the distribution of viruses on cucurbits in South Georgia and will be useful to develop strategies for managing the complex of whitefly-transmitted viruses in the region.