Insight into Biological Control Potential of Hirsutella citriformis against Asian Citrus Psyllid as a Vector of Citrus Huanglongbing Disease in America

Abstract

Studies on Hirsutella citriformis Speare are scarce. Among these, some reports have focused on phenotypic identification, based on its morphological structure and morphometric characteristics. This fungus is known to control economically important citrus crop pests. In recent years, H. citriformis has received increased attention as a control agent for the Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae), which causes the Huanglongbing (HLB) disease. Unfortunately, formal H. citriformis strains characterization is marginal, which mainly involves the role of biologically active exudates (metabolites) produced during their growth. Information regarding their mode of action and biocontrol potential is limited. However, epizootics reports of this fungus, under suitable environmental conditions for its development (25 °C to 28 °C and ~80% relative humidity), have demonstrated its parasitization efficacy. Therefore, it becomes challenging to determine whether H. citriformis strains may be developed as commercial products. In this review, we showed relevant information on isolation and bioassay strategies of H. citriformis to evaluate potential biocontrol strains under laboratory and field conditions in America.

Keywords: Diaphorina citri; Huanglongbing (HLB) disease; biocontrol agents; entomopathogenic fungi; insect parasitization; metabolites production.

Figure 1

Hirsutella citriformis mycelium structures. (A) H. citriformis synnemata developing on Diaphorina citri and (B) phialide and conidia. Measurements are in micrometers.

Figure 2

Mexican Hirsutella strains morphological characteristics. (A) Phialide length, (B) conidium width, (C) conidium length, (D) conidium width, including the mucilaginous envelope, and (E) conidium length, including the mucilaginous envelope. Measurements are in micrometers.

Figure 3

Hirsutella thompsonii mode of action.

Figure 4

Different exudates produced by Mexican Hirsutella citriformis strains grown on potato dextrose agar supplemented with 0.1% yeast extract (PDAY). (A) H. citriformis strain producing mostly light brown exudates and (B) H. citriformis strain producing (a) deep brown, (b) crystalline, and (c) light brown exudates.

Figure 5

Polyacrylamide gel electrophoresis of a Mexican Hirsutella citriformis strain sample. Lane 1 = mycelium; lane 2 = supernatant; lane 3 = mycelium; lane 4S = gummous material dissolved in solvent and sonicated (five pulses/40 W/30 sec). This material was slightly fragmented; lane 4C = the gummous material was cut, and a fragment was directly deposited as sample; lane 5 = solvent. Note that samples dissolved in a solvent were off the gel lane.