Abstract
ABSTRACTFusarium spp., which are common inhabitants of oil palm leaves, are weak pathogens of common spear rot (CSR). We investigated the influence of osmotic stress on the growth, virulence, and activity of cell wall-degrading enzymes of CSR fungi, using potato dextrose agar (PDA) supplemented with KCl or sucrose (hyperosmotic medium). Hyperosmotic stress significantly inhibited mycelial growth, but growth rapidly recovered when mycelia were transferred to control medium. When inoculated into oil palm spear leaflets, Fusarium sp., and F. incarnatum precultured on 1.0 and 1.5 M KCl-hyperosmotic medium induced lesions that were two to four times larger than those in non-stressed cultures, suggesting enhanced virulence of the weak pathogens. Lesion size was not greatly affected in hyperosmotic cultures of moderately virulent F. sacchari. No activity of pectin lyase was detected in liquid cultures of the Fusarium isolates. All isolates except F. incarnatum BT48 secreted polygalacturonase (PG), which was active in both liquid cultures and inoculated leaves. Significantly increased PG activity (5–32-fold) was observed on leaves inoculated with hyperosmotic cultures of Fusarium sp. and F. sacchari. These findings suggest that Fusarium sp., F. incarnatum, and F. sacchari exhibit an adaptive physiological plasticity to hyperosmotic stress that results in enhanced virulence.
Highlights
Common spear rot (CSR), known as crown disease, is the most common disease in young oil palm (Elaeis guineensis Jacq.) worldwide (Breure & Soebagjo 1991; Chinchilla et al 1997; Corley & Tinker 2003)
Mycelial growth was suppressed under hyperosmotic stress, the growth of isolate BT48 was extensive on potato dextrose agar (PDA) supplemented with 0.6 M KCl
This study demonstrated that Fusarium species, which are weak pathogens of oil palm spear rot, can rapidly recover from drought and salt stress
Summary
Common spear rot (CSR), known as crown disease, is the most common disease in young oil palm (Elaeis guineensis Jacq.) worldwide (Breure & Soebagjo 1991; Chinchilla et al 1997; Corley & Tinker 2003). The disease has caused considerable losses of young oil palms, but few detailed studies of the pathogenic agent of the disease have been conducted. Our study (Suwandi et al, 2012) suggested that Ceratocystis paradoxa, Fusarium sacchari, and weakly virulent fungi dominated by Fusarium spp. were associated with oil palm CSR in Indonesia. C. paradoxa and F. sacchari are strongly and moderately virulent, respectively, but their prevalence was low. Fusarium species are considered weakly virulent as they induce only a very small lesion under 5 mm in length on the normal oil palm leaves
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