Sweet potato feathery mottle virus (SPFMV) is a serious concern to the sweet potato [Ipomoea batatas (L.) Lam.J industry in Queensland. The Queensland Department of Primary Industries (QDPI) operates a costly program for maintenance of sweet potato germplasm and provision of virus-tested planting material. Justification for continuing this program is largely dependent on determining whether or not SPFMV causes significant losses in yield.
Disease symptomatology, virus detection methods, effects of different isolates of SPFMV on foliar growth, yield and other characteristics of commercially grown cultivars, sweet potato tissue culture and yield performance of tissue cultured plantlets were studied.
A single isolate of SPFMV was collected from South-East Queensland (SPFMV-SEQ), Northern Queensland (SPFMV-NQ) and northeast New South Wales (SPFMV-NSW). Each isolate was graft-transmitted
into plants of commercially important cultivars. The results showed that symptom development is highly dependent on cultivar, isolate of SPFMV. growth stage of plants and environmental conditions such as temperature and light intensity. Symptoms of internal cork and russet crack were not observed.
A comparison of virus detection methods indicated that membrane immunobinding assay (MDBA) and graft indexing (Ipomoea setosa) were very sensitive if appropriate tissue was used. MIBA is the preferred detection method, in terms of efficiency, sensitivity and cost. Studies on the effects of SPFMV-SEQ on yield and other characteristics of cvs LO-323, NC-3, Beerwah Gold and Coleambally showed that SPFMV-SEQ affected these cultivars similarly. Fresh marketable yield was reduced (P<0.05) by 13.6% (2.9 t/ha) at harvest. The yield reduction was mainly due to fewer tuberous roots rather than smaller tuberous roots. Foliar growth (except for total
chlorophyll content), dry matter content of tuberous roots, fresh and dry biological weights, and storage quality (except for the loss in total fresh weight of tuberous roots) were not affected (P>0.05).
A comparison of infected material of two different ages (cv NC-3) showed that material obtained from infected plants propagated vegetatively over several seasons resulted in significant losses (P<0.05) in total yield (12.4% or 4 t/ha) and marketable yield (17.5% or 4.3 t/ha), whereas the recently graft infected material did not cause significant yield losses (P>0.05). This suggests that growers should use virus free (tested) planting material to avoid built-up of the virus.
Studies on the effects of different isolates on growth and yield of the cultivars LO-323 and Beerwah Gold showed that the three regional isolates affected the cultivars similarly. There was little difference between the two Queensland isolates
(SPFMV-SEQ and SPFMV-NQ), in terms of the effects on growth and yield. Both isolates significantly reduced (P<0.05) total and marketable yields of tuberous roots by 15.6-19.7% (8.4-10.4 t/ha) and 18.9-22.7% (8.8-10.6 t/ha) respectively, when compared to healthy material. Neither of the two isolates significantly affected (P>0.05) average weight of tuberous roots, weight and number of unmarketable tuberous roots, fresh and dry weights of plant tops, dry matter content of tuberous roots and plant tops, or total soluble solids content of tuberous roots.
The isolate from New South Wales (SPFMV-NSW) caused significant reductions (P<0.05) in total and marketable yields of tuberous roots, accounting for 74.9% (39.5 t/ha) and 79.4% (37 t/ha) respectively, as compared to healthy material. The average weight of tuberous roots was also reduced significantly (P<0.05). Dry matter content of plant tops and total soluble solids content of
tuberous roots were significantly increased (P<0.05) due to infection with SPFMV-NSW. Further testing confirmed that SPFMVNSW was a complex of SPFMV and a phytoplasma.
Tissue culture studies demonstrated that most plantlets (72%) survived on a long-term storage medium for one year without transfer. Fresh yield and number of total, marketable and unmarketable tuberous roots produced by the tissue cultured plantlets grown in the field were not dissimilar to those recorded from traditional shoot cuttings. The studies indicated that it would be possible for QDPI to establish a new program using tissue culture techniques for maintenance of sweet potato germplasm and production of virus free planting material.