William Otim-Nape

1. An epidemiological model of African cassava mosaic virus disease (ACMD) was developed in which the dynamics within a locality, of healthy and infected cassava, and of infective and non-infective whitefly vectors, were specified. 2. Cassava is propagated ...

A feature of the production of cassava (Manihot esculenta Crantz) in Uganda, as in many other countries of subSaharan Africa, is that many different varieties are grown and it is usual to find several varieties within individual plantings. Moreover, cassava is usually grown together with one or more intercrops, including legumes and cereals. The great diversity in the varieties grown and in the cropping systems adopted can be expected to influence the incidence and severity of cassava pests and diseases. However, this possibility has received only limited attention in relation to cassava mosaic disease (CMD), despite indications that the current pandemic in Uganda and in adjacent parts of north-west Tanzania and western Kenya is closely associated with the varieties adopted and is least damaging in areas where many varieties are grown.

A stock culture of cotton leaf curl virus from Pakistan (CLCuV-PK), was transmitted by whiteflies (Bemisia tabaci) to seven plant species, including French bean, okra, tobacco and tomato, and caused vein thickening and leaf curl symptoms. It was readily detected in triple antibody sandwich ELISA (TAS-ELIS A) by 11 out of 31 monoclonal antibodies raised against the particles of three other geminiviruses: African cassava mosaic, Indian cassava mosaic and okra leaf curl viruses. Reaction strength was enhanced when the tissue extraction fluid contained sodium sulphite. Minor variations in epitope profile were found among virus isolates from cotton (Gossypium hirsutum) collected from different districts in Pakistan over a 5-year period. These epitope profiles were distinguishable from that of cotton leaf curl virus from G. barbadense in southern India but indistinguishable from the profiles of viruses causing yellow vein disease of okra in India or Pakistan, or leaf curl of okra {Abelmoschus esculentus), Hibiscus tiliaceus, radish or sunflower in Pakistan, suggesting that these plants are putative natural hosts of CLCuV-PK. The viruses in cotton, and in okra with leaf curl or yellow vein symptoms, were also detected by PCR with three pairs of CLCuV-PK-specific primers.Five additional whitefly-transmitted geminiviruses were found among isolates from 11 other naturally-infected species in Pakistan, and were distinguished by their epitope profiles. These viruses were associated, respectively, with tobacco leaf curl, squash yellow blotch, tomato yellow leaf curl, watermelon leaf crinkle and soybean yellow mosaic diseases. The first four of these viruses were detected readily by PCR with geminivirus general primers but only weakly, if at all, with two pairs of CLCuV-PK-specific primers. Pakistani crops are infected with a range of distinguishable but relatively closely related whitefly-transmitted geminiviruses, some of which resemble those found in India.

The spread of cassava mosaic disease (CMD) in a range of cassava cultivars was studied in experiments and on-farm trials in different agro-ecological regions of Uganda in 1989–1990 and 1990–1991. No spread occurred in either experiment at the southernmost site near Kampala, but there was considerable spread at the four sites elsewhere and also in the on-farm trials in Luwero district. There were significant differences in the final incidence of disease between locations and between cultivars at each location. Where spread occurred it was more rapid in the Ugandan cvs Ebwanateraka, Senyonjo and Bao than in four of the five improved TMS cultivars introduced from Nigeria. These usually showed an apparent decline in incidence of CMD after reaching maxima 4 to 8 months after planting (MAP). The areas under the disease progress curves (AUDPCs) differed significantly between locations and cultivars and were less for cvs TMS 30572, TMS 30395, TMS 30337 and TMS 60142 than for cvs Ebwanateraka, Senyonjo, Bao and TMS 30786. Overall, the mean AUDPCs were greatest at Migyera in Luwero district in 1989–1990 and at Kagando in Kasese district in 1990–1991. They were significantly less at Mubuku in Kasese district in 1989–1990 than at the other two experimental sites where spread occurred. Adult whitefly vector populations were highest at Migyera and Kagando in the 1989–1990 and 1990–1991 trials, respectively, and they were higher on cvs Bao, Ebwanateraka and TMS 30786 than on other varieties. Mean numbers of adults increased until 3–5 MAP and then declined, but CMD incidence increased progressively to reach maxima at or near crop maturity. Locations with the largest numbers of adults also had a relatively high incidence of CMD. Symptoms of CMD were usually more severe on cvs Ebwanateraka, Bao and Bukalasa 11 than on the TMS cultivars, on which symptoms remained slight throughout growth and usually decreased from 5 MAP. The differences between sites, the resistance of the cultivars and the relationship between CMD incidence and whitefly populations are discussed.

Mosaic disease (MD) is more severe in cassava plants infected within the area of the current epidemic in northern and central Uganda than to the south of the affected area. This difference in severity was recorded within a single cultivar as well as amongst the mixtures of cultivars found commonly in farmers' fields. An increase in severity also occurred as the epidemic passed through localities. Varietal or agroecological factors coincident with the area of the epidemic are therefore unlikely to cause the increased severity. The severe disease could also be graft and cutting transmitted and could super-infect mildly diseased plants. Both mildly and severely diseased plants gave positive reactions in ELISA tests to antisera prepared against African cassava mosaic virus (ACMV) and an unusually severe form of ACMV or a closely related geminivirus is likely to be the cause of the severe mosaic disease. The epidemic also involves increased populations of the whitefly vector of ACMV, Bemisia tabaci, and possible hypotheses are presented as to how these phenomena may be related.

ABSTRACT During November 2004, veinal chlorosis on mature cassava leaves, typical of cassava brown streak disease (CBSD), was observed at Mukono in central Uganda. Five out of 11 cultivars at the site showed CBSD symptoms (incidence range 4 to 64%). In a survey ...

The effects of cassava mosaic virus disease (CMD) on yield in fully and partly infected stands of cassava were investigated in field trials in Uganda in 1990-91 and 1991-92. Three cultivars (Ebwanateraka, Bao and Bukalasa 1 l), each at three levels of cutting infection (O%, 50% and 100%) and harvested 510 and 15 months after planting (MAP) were used in a randomised block design with split-split plots and four replicates. Moreover, yield and growth data for individual infected and uninfected plants were considered in relation to the health status of their nearest neighbours. In each experiment, fresh tuberous root yields of plants from 100% infected plots gave sigdicantly lower yields than those from 0% or 50% infected plots at each harvest date and the losses were greatest in cv. Bao. Yields of plants from 0% and 50% plots for each of the three cultivars were not significantly different, 10 and 15 MAP. The loss in yield differed between cultivars and harvest dates. Fresh stem, leaf and root yields and the number of tuberous roots were influenced by the health status of the plants harvested and that of their nearest neighbours. Uninfected plants surrounded by infected ones had more roots and heavier total fresh root, stem and leaf weights than those surrounded by uninfected ones. Overall, 26% and 42% compensation was recorded in 1990-91 and 1991-92, respectively. The effects of CMD on cassava production and of compensation in mixed stands of infected and uninfected plants are discussed, especially in relation to control strategies such as roguing.

To study the cause of the current epidemic of severe mosaic in Ugandan cassava, PCR analysis was used to detect and identify African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV) and the recently reported recombinant geminivirus (UgV), which is derived from ACMV and EACMV, in leaf extracts from cassava plants grown from cuttings in the glasshouse at Dundee. The cuttings were collected from plants showing symptoms of different severities and growing at different sites in Uganda inside, at the periphery of, and outside, the area affected by the epidemic. ACMV occurred throughout the nine districts sampled but UgV was detected only in the area affected by the epidemic. EACMV was not found in Uganda. Most plants containing ACMV alone expressed mild or moderate mosaic, whereas very severe mosaic developed in most plants containing UgV plus ACMV and a few of those containing UgV only. Very severe mosaic in cassava from southern Sudan was likewise associated with co-infection by UgV and ACMV. The very severe disease was reproduced by graft-inoculating geminivirus-free cassava with UgV plus ACMV; plants inoculated with either UgV or ACMV developed severe or moderate symptoms, respectively. Unlike ACMV, Malawian EACMV did not enhance the severity of symptoms induced by UgV. However, a very severely affected plant from Ukerewe Island, Tanzania, contained ACMV and EACMV but not UgV. UgV attained a much greater concentration in cassava than did ACMV but the opposite occurred in Nicotiana benthamiana. In neither host was total virus antigen concentration affected by co-infection.Factors affecting the genesis, selection and spread of UgV are discussed. The evidence indicates that UgV is probably of relatively recent origin, that such variants do not appear often, and that the current epidemic has resulted from the rapid spread of UgV to infect plants and to invade regions in which ACMV already occurred. The novel type of virus complex so produced, consisting of an interspecific recombinant virus (UgV) and one of its parents (ACMV), typically has even more severe effects than UgV alone.

A survey in 1994 assessed the incidence and severity of Cassava mosaic virus disease (CMD) in Uganda and the changes that had occurred since the previous survey in 1990–92. Three counties in each of 27 districts were selected and 15 fields were assessed per county. CMD occurred in each of the 1215 fields surveyed and the overall incidence within fields was 64% compared with 52% in the same districts in 1990–92. Mean incidence differed significantly between districts and between counties within some districts. The incidence and severity of CMD had increased in 20 and 23 of the 27 districts, respectively, compared with 1990–92, indicating the continued progress of the epidemic that was first reported in Uganda in 1988. In all but three districts, there had been a decrease in the total area of cassava. The number of different varieties that predominated in one or more fields had increased in 17 districts compared with 1990–92, although the total number of varieties encountered was little changed. Ebwanateraka was again the most widely grown variety but it was recorded less frequently and predominated or was co-dominant in only two districts in 1994 compared with seven in 1990–92. The biggest decrease in the cultivation of Ebwanateraka had occurred in districts where there was a high incidence of CMD in 1990–92.

The severity and persistence of symptoms of mosaic virus disease were monitored during the first six months of two growing seasons in cassava of the African cassava mosaic virus (ACMV)-resistant cv. TMS 30572 either inoculated by grafting with a mild or severe strain or infected from the planted cutting. Symptomless shoots developed between January and March 1995 in two field trials differing in age by c. 6 months; this recovery occurred during particularly hot weather. Recovery was often only temporary in the plants inoculated with the severe strain and occurred later compared with those inoculated with the mild. In 1996, the weather was cooler and recovery that year was delayed until flowering, c. 7 months after planting, when recovered shoots were often produced from buds in the axils of symptomless leaves produced amongst diseased leaves. Most cuttings taken from the upper parts of diseased plants produced symptomless (reverted) progenies whereas most cuttings taken from the base of diseased plants produced diseased progenies. Reversion seemed to be associated with the recovery that had already occurred in the upper stems of the parent plants.

Progress curves of cassava mosaic virus disease (CMD) and populations of the whitefly vector (Bemisia tabaci) were assessed using four cassava varieties grown alone and as a random mixture in two experiments established under epidemic conditions at a site near Kampala in southern Uganda. There were significant differences in final CMD incidence and in the areas under the disease progress curves between varieties when grown alone and as a mixture in both experiments. Variety Ebwanateraka had the highest incidence and SS4 the lowest, even though it supported the largest populations of adult whiteflies. The overall incidence of CMD in the mixture was similar to that in pure stands of the partially resistant Nase 2 and greater than in the resistant Migyera and SS4. Compared to pure stands, incidence of CMD in each component of the mixture was reduced significantly only in Ebwanateraka, whereas vector populations were less only in SS4 and Nase 2. On several observation dates the actual incidence of CMD and populations of adult whiteflies in the mixture were significantly less than expected values estimated from the results for the four varieties when each was grown alone. A highly significant positive relationship was established for each variety between peak populations of adult whitefly and leaf area index at the time. The implications of the findings and the scope for future research on the use of varietal mixtures for the management of CMD are discussed.