Coat protein of partitiviruses isolated from mycorrhizal fungi functions as an RNA silencing suppressor in plants and fungi.

Orchid seeds depend on colonization by orchid mycorrhizal (OM) fungi for their germination; therefore, the orchids and OM fungi have long maintained a close relationship (e.g., formation of the hyphal mass structure, peloton) during their evolution. In the present study, we isolated new partitiviruses from OM fungi; partitivirus were separately found in different subcultures from the same fungi. Partitiviruses have been believed to lack an RNA silencing suppressor (RSS), which is generally associated with viral pathogenicity, because most partitiviruses isolated so far are latent in both plants and fungi. However, we found that the coat protein (CP) of our partitiviruses indeed had RSS activity, which differed among the virus isolates from OM fungi; one CP showed RSS activity in both plants and fungi, while another CP showed no activity. The family Partitiviridae include viruses isolated from plants and fungi, and it has been suggested that these viruses may occasionally be transmitted between plant and fungal hosts. Given that there are several reports showing that viruses can adapt to nonhost using strong RSS, we here discussed the idea that partitiviruses may be better able to migrate between the orchid and fungus probably through the pelotons formed in the orchid cells, if host RNA silencing is suppressed by partitivirus RSS.

Quantification of Phytophthora infestans population densities and their changes in potato field soil using real-time PCR.

Tuber infection of Phytophthora infestans often occurs at harvest. However, it is difficult to accurately estimate the population densities of P. infestans in soil, especially Japanese soil. In the present study, P. infestans DNA was extracted from soil samples using a modified CTAB-bead method and quantified using real-time PCR to accurately, rapidly and easily estimate the P. infestans population densities in upland soils in Japan. P. infestans was well quantified in eleven types of soil samples, including nine types of upland soils in Japan, that were artificially inoculated with a zoosporangia suspension. The amounts of P. infestans DNA estimated by the real-time PCR were proportional to the inoculum densities. In the non-controlled experimental potato field, P. infestans population densities in soil corresponded to the development of symptoms and were correlated with the number of lesions on the potato foliage. These results imply that the proposed real-time PCR assay is suitable for the estimation or monitoring of P. infestans population densities in upland soils in Japan. The population densities at the ridge bottoms were larger than those at any other location in commercial potato fields. These results were similar to those of a previous report using a bioassay. Moreover, a correlation between DNA quantity and inoculum potential was observed. In conclusion, the real-time PCR assay developed in this study is suitable for indirect estimation of the inoculum potential of P. infestans.

Enhanced virulence of Fusarium species associated with spear rot of oil palm following recovery from osmotic stress.

Fusarium 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.

Common Spear Rot of Oil Palm in Indonesia.

Common spear rot (CSR), which is also known as crown disease, was first reported in Indonesia in the 1920s. It has caused considerable losses in young oil palm plantings, and yet the pathogenic agent has remained elusive. Symptomatic spear leaves were collected from oil palm plantations and farm plots in South Sumatra, North Sumatra, and Bangka-Belitung, Indonesia. Of the 14 different fungi isolated, Fusarium incarnatum, F. solani, an undescribed Fusarium sp., and Ceratocystis paradoxa were isolated most frequently from diseased leaf tissue. F. incarnatum and the undescribed Fusarium sp. were also frequently isolated from healthy leaf tissue, along with Pestalotiopsis microspora and Curvularia affinis. Ceratocystis paradoxa was never isolated from healthy leaf tissue. Koch's postulate experiments showed that C. paradoxa was able to infect wounded oil palm leaves causing a symptom of extensive rotting similar to that found in the field. Although isolated less frequently and less virulent than C. paradoxa, F. sacchari was also capable of causing lesions on succulent wounded, inoculated leaves. For both C. paradoxa and F. sacchari, the disease severity index was greater when the oil palm leaves appeared to have more succulent growth. Likewise, other Fusarium species and other nonfusarial fungi that were usually not pathogenic were weakly virulent on palms with more succulent growth. These findings confirm that C. paradoxa is one pathogen that is associated with CSR of oil palm in Indonesia.