No abstract

Sugarcane, Saccharum spp. hybrid, is widely infected in the United States and many other countries with a yellowing and stunting disease called sugarcane yellow leaf syndrome. The causal agent, Sugarcane yellow leaf virus (ScYLV), is a Polerovirus of the Luteoviridae family. In this study, it was transmitted by the sugarcane aphid, Melanaphis sacchari, and also by the corn leaf aphid, Rhopalosiphum maidis, and the rice root aphid, R. rufiabdominalis. Two other aphids that infest sugarcane in Hawaii did not transmit the virus. Some Hawaiian sugarcane cultivars are susceptible to ScYLV, while others remain virus-free in the field. The latter were not infected when inoculated with viruliferous M. sacchari. Virus-free plants of susceptible cultivars were produced through apical meristem culture and were readily reinfected by viruliferous M. sacchari. They were also quickly reinfected when planted in a field in proximity to other infected sugarcane naturally infested with M. sacchari. Sugarcane cultivars are hybrids of several Saccharum species. In a field-grown collection of Saccharum and related species, 11 to 71% of the clones of four of the species were infected with ScYLV. None of the related genus Erianthus plants were infected, but four clones were infected experimentally by aphid inoculation. A low to moderate percentage of corn, rice, and sorghum seedlings became infected when inoculated with ScYLV, but barley, oats, and wheat proved to be very susceptible. None of seven weeds common in sugarcane fields were infected with ScYLV.

Sugarcane yellow leaf virus (SCYLV) has been reported worldwide to infect sugarcane and to cause significant yield losses. Current detection methods include tissue blot immunoassay (TBIA), reverse transcription-polymerase chain reaction (RT-PCR) and quantitative real-time RT-PCR assay (qRT-PCR). In this paper, we report the use and comparison of these detection methods for the study of SCYLV in Hawaiian cultivars. We observed positive RT-PCR and qRT-PCR reactions in cultivars previously thought (based on TBIA) to be immune to virus infection. The semi-quantitative virus titre in these cultivars was however at least 10 6 -fold lower than in the cultivars which were known to be SCYLVsusceptible. The RT-PCR methods also revealed that plants of the cultivar H65-7052, which were previously shown to vary strongly between TBIA-positive and TBIA-negative, indeed exhibited fluctuating SCYLV-titres in a range of 10 3 -10 4 -fold. The virus titre was carried through to the next vegetative generation, i. e. plants grown from seed pieces with low virus titre had low virus titre and plants grown from seed pieces with high virus titre contained high virus titre. A small field trial comparing plants of cv. H65-7052 of low and high SCYLV-titre showed that the field plots with plants of high virus titre developed Yellow Leaf symptoms and yielded only 54-60% of cane and sugar tonnage compared to plots with plants of low virus titre.

Sugarcane yellow leaf virus (ScYLV) is distributed worldwide and has been shown to be the cause of the disease sugarcane yellow leaf syndrome (YLS). This study was an investigation of the transmission and spread of ScYLV in Hawaii. Several aphids are known to transmit the virus, but investigation of infestation and transmission efficiency showed Melanaphis sacchari to be the only vector important for field spread of the disease. The initial multiplication of ScYLV in a virus-free plant occurred exclusively in very young sink tissues. When a single leaf was inoculated on a plant, that leaf and all older leaves remained virus-free, based on tissue-blot immunoassay, whereas meristems and all subsequently formed new leaves became infected. Therefore, only after those leaves which had already developed before inoculation had been shed, did the complete plant contain ScYLV. Spread of the viral infection to neighbouring plants in the plantation fields via aphids was relatively slow and in the range of a few metres per year. No indication of long-distance transfer could be seen. This indicates that it may be possible to produce and use virus-free seed cane for planting of high-yielding but YLS-susceptible cultivars.

Thirty-eight strains of the sugarcane leaf scald pathogen, Xanthomonas albilineans, were compared using a series of seven species-specific monoclonal antibodies as well as genomic DNA fingerprint patterns. The strains, which were obtained from 13 countries and states in the USA, were placed in three major groups with approximately eight subgroups based on similarity of their serological reactions. The serological groupings correlated strongly with groupings based on DNA fingerprint patterns. The groups of strains that were genetically and serologically related did not necessarily come from nearby locations but were usually from widely separated regions. Thus, it appears that disease transmission occurred by means other than natural spread. The methods used in this study are useful for comparing relationships among strains of X. albilineans. A combination of the monoclonal antibodies could be used in a serodiagnostic-based method to test for the presence of leaf scald disease.