Identification of a new Cordyceps javanica fungus isolate and its toxicity evaluation against Asian citrus psyllid

Abstract

The Asian citrus psyllid, Diaphorina citri Kuwayama, is the most serious pest of citrus worldwide. It acts as a vector for a group of phloem-limited bacteria (Candidatus Liberibacter spp.) that causes Huanglongbing (HLB) disease. Thus, D. citri management is an important strategy against HLB, and biological control is currently considered as the most effective method because of the unsustainable and negative side effects of chemical control. Here, we isolated a new strain of entomopathogenic fungus, Cordyceps javanica (GZQ-1), from one cadaver of D. citri adult based on its morphological and phylogenetic data. Five conidial concentrations of the C. javanica pathogen (1 × 10³ , 1 × 10⁴ , 1 × 10⁵ , 1 × 10⁶ , and 1 × 10⁷ conidia/ml) were assessed against six life stages of D. citri (1st-5th instar nymphs and adults). Results showed that C. javanica GZQ-1 was highly pathogenic to D. citri nymphs (69.49%-90.87% mortality) and adults (69.98% mortality). The LC₅₀ and LT₅₀ values of C. javanica against 1st-2nd instar (younger), 3rd-4th instar (middle aged), 5th instar (older), and adults were 1.20 × 10⁵ , 1.10 × 10⁶ , 4.47 × 10⁶ , 8.12 × 10⁶ conidia/ml and 4.25, 4.51, 5.17, 5.49 days, respectively. Moreover, glasshouse experiments indicated that this C. javanica GZQ-1 caused higher infection rates of D. citri adults compared to two other fungal strains we previously isolated in the laboratory, Cordyceps fumosorosea (IF010) and Metarhizium anisopliae (CNGD7).

Keywords: Cordyceps javanica; Asian citrus psyllid; biological control; entomopathogenic fungi; isolation.

Figure 1

The morphology of Cordyceps javanica GZQ‐1 isolate on SDAY/4. (a) The upper side of C. javanica colony on SDAY/4 on 4th day; (b) The upper side of C. javanica mature colony on SDAY/4 on 14th day; (c) C. javanica GZQ‐1: chains of conidia. (d) C. javanica GZQ‐1: Phialides with developing conidia

Figure 2

The infection phenotype of Diaphorina citri nymphs treated with Cordyceps javanica (1 × 10⁷ conidia/ml). Panels a, b, and c were the 1st, 2nd, and 3rd nymphs of D. citri on 2nd day after infection while panels d, e, and f were the 1st, 2nd, and 3rd nymphs on the 7th day after infection

Figure 3

The infection phenotype of Diaphorina citri older nymphs and adults treated with Cordyceps javanica (1 × 10⁷ conidia/ml). Panels a, b, and c were the 4th and5th nymphs and mature adults of D. citri on 2nd day after infection while panels d, e, and f were the 4th and 5th nymphs and mature adults on the 7th day after infection

Figure 4

Majority rule consensus phylogram from the Bayesian analysis based on the sequences of the internal transcribed spacer (ITS) region for 18 isolates (GZQ‐1 isolate, Cordyceps javanica, Cordyceps fumosorosea, Beauveria brongniartii, Beauveria bassiana. Metarhizium anisopliae was used as the outgroup). Support values are shown for ML BS. The bars in different color show the different fungal species but different strains clustered into one branch in the phylogenetic tree

Figure 5

The mortality of Diaphorina citri different developmental stages treated with various concentrations of Cordyceps javanica (1 × 10³, ×10⁴, ×10⁵, ×10⁶, and ×10⁷ conidia/ml). Data are mean ± SEM of three tests

Figure 6

The survival rates of Diaphorina citri adults when infected with Cordyceps javanica GZQ‐1 isolate as well as Cordyceps fumosorosea and Metarhizium anisopliae (all are 1 × 10⁷ conidia/ml) under glasshouse conditions. Data are mean ± SEM of three tests