Search for a command to run...
In August 2024, approximately 12% of the 513 plots in a 3-acre raspberry test plot in Santa Cruz County showed new disease symptoms, with some plots having 100% mortality. Symptoms included chlorotic and necrotic leaves, a longitudinal black streak on one side of the cane, cane blighting, and entire plant death. White to pink sporodochia were observed on older cane lesions, often localized near petiole attachments. Plants were sampled from symptomatic plots and affected crown and cane tissues were surface disinfested and cultured on acidified ½ strength PDA (APDA). Tissues were tested for Verticillium dahliae and Phytophthora species by qPCR (Bilodeau et al., 2012; Miles et al., 2017). Tissues from 5 plots tested positive for P. bishii . All samples on APDA produced fluffy white colonies with pink coloration on the agar underside. Microconidia (3.3-8.5 μm × 1.5-3.5 μm) were ellipsoidal and non-septate, arising from monophialides. Monosporic pure cultures were generated from two colonies isolated from crown (24-119ss) and cane (24-136ss) tissue. Translation elongation factor 1-alpha (EF1α) and RNA Polymerase II subunit B2 (RPB2) were amplified, and sequenced with Sanger sequencing, using EF1/EF2 and 5f2/7cr in combination with 7cf/11ar primers, respectively (O’Donnell et al., 1998, 2022). The two isolates differed from each other by two SNPs in the EF1α sequences; however, both isolates were 100% identical to Fusarium oxysporum f. sp. fragariae (Fof). Isolate 24-119ss (GenBank PV764553) was identical to GL1080 (GenBank KX456097), and 24-136ss (GenBank PV764555) to GL1315 (GenBank KX456064). The concatenated RPB2 (GenBank PV764554, GenBank PV764556) sequences of both isolates yielded a 99% match to Fof (GenBank KX434986). Both isolates were also confirmed as Fof by a positive qPCR amplification (Burkhardt et al., 2019). Pathogenicity of each isolate was confirmed by dipping roots of raspberry tray plants into a suspension of 106 conidia/mL for 30 minutes. Six proprietary cultivars were inoculated with 8 replicates per isolate; 2 non-inoculated plants were used as controls. After 4 weeks, severe wilting and collapse was observed in 5 cultivars inoculated with these 2 isolates, whereas the control plants remained healthy. Colonies identical in morphology and EF1α/RPB2 sequences were then recovered from symptomatic cultivars. A second experiment was performed to test the virulence on both strawberry and raspberry of a known Fof race 1 isolate, GL1059 (Henry et al., 2017), and the raspberry isolate 24-136ss. Two cultivars (a known resistant and a known susceptible) per crop were inoculated as frozen dormant plants using the method above, with 5 replicates per isolate per cultivar. Ten weeks after inoculation, severe wilting and collapse was observed in susceptible cultivars of raspberry inoculated with GL1059, and in strawberry inoculated with 24-136ss, respectively. The resistant strawberry cultivar, which carries FW1, was resistant to the raspberry-derived isolates, confirming these were Fof race 1 (Henry et al., 2021). The cross-infectivity of isolates, along with their genetic relatedness to Fof, demonstrate that the causal organism of Fusarium wilt in raspberry is Fusarium oxysporum f. sp. fragariae race 1. Expansion of the Fof host range to include raspberry may cause significant nursery and fruit production losses worldwide and disrupt crop rotation strategies.