Viruses and pathogens are wreaking havoc on yam plants, nearly omnipresent. In addition to being physically transmitted via rainfall, insects, humans, machinery, etc., these viruses persist generationally whether yams are cultivated through through diseased cuttings or even transmission in seeds (Kumar/Ayodeji). A survey of Nigeria and Ghana, producers of 90% of the worlds’ yam supply, was conducted for incidence of the three most economically-devastating yam viruses and the fungus, Colletotrichum gloeosprioides, which causes anthracnose disease. Yam mosaic virus, yam mild mosaic virus, and Dioscorea bacilliform virus, were present 95% and 92% of the time, respectively in Ghana and Nigeria. Meanwhile, C. gloeosporioides was present in 72% and 77% of cases, though anthracnose disease was not nearly as prevalent at a mere 9% (Nkere).
The following is a list of viruses infecting yam sorted by genus:
While this site is under construction, please follow these links for more details:
ELISAs (enzyme-linked immunosorbent assay) and RT-PCRs (Reverse Transcription Polymerase Chain Reactions) are the prevailing diagnostic tools for plant virus diagnostics. Thus far, prerequisite antibodies for ELISA specific respectively to YMV, YMMV, and CMV—the major yam viruses—have been developed (Hema). Further, a low-cost, robust multiplexing RT-PCR method has been developed such that all three diseases can be tested for in tandem using leaves, vine cuttings, in vitro plantlets, tubers, or leaf sap (Nkere). To enhance accessibility of viral testing for farmers, a method of preserving yam leaf samples, namely in TRIS buffer (pH 8) containing CTAB, for up to 38 days has been validated (Nkere). Alternately, RT-exoRPA (reverse transcription-recombinase polymerase amplification) has also showed promised as a less intensive though equally accurate/precise method of detection for YMV that potentially could be pushed into the field.
The most effective method of viral treatment is to go straight to the source: the ‘seed’ yam. However, whether an ‘seed’ yam or a truly-bred seed, viruses can be transmitted to the offspring. Thus, preemptively it is best to propagate clean seed. In tissue cultures, Jeeva et. al. were able to eliminate YMMV thermo-therapeutically in 70% of subsequent plants with hot water treatment at 36℃ for 30 minutes; and 90% with combined chemo-thermotherapy (i.e. 10mg/L Quercetin) and thermotherapy (36℃ for 45 days).
Fungal pathogens are a severe and widespread problem. Anthracnose disease, caused by Colletotrichum gloeosporioides, is one of the most devastating diseases—especially to D. alata and D. dumetorum—with Petro (2013) reporting up to 85% yield losses [REF]. Other commonly reported fungal pathogens of yam include Aspergillus niger, Asperigillus flavus, Rhizopus spp., Rhizoctonia solani, Penicillium oxalicum, Sclerotium rolfsii, Botriodiploida theobromae, Lasiodiplodia theobromae, Fusarium oxysporum, and F. monoliform. Not only reducing yields, these pathogens also reduce yams’ shelf-life and nutrition.