Short communication . Cotton cultivars with moderate resistance to African cotton mosaic virus

A 3 year screening of 25 long staple cotton cultivars for resistance to African cotton mosaic virus (ACMoV) conducted in northern Nigeria during the 1999-2001 wet seasons indicated that no cultivar was immune or resistant. Two cultivars, Pima S1 and Pima S4 were moderately resistant (16% infection), four cultivars Pima S2, Pima S3, Giza 45 and Giza 69 were moderately susceptible (31% infection), while the remaining 19 cultivars were highly susceptible. The moderately resistant cultivars are high yielding, have high quality lint characteristics such as crin percentage, bundle strength, and micronaire fineness. They can be used to reduce the menace caused by ACMoV on cotton in Nigeria. Additional key words: disease resistance, Gossypium hirsutum cultivars, plant virus.

that the incidence of the virus ranged from 4.1 to 12.0% (Alegbejo, 1999).The most effective, sustainable and environmentally friendly control measure against plant viruses is the use of resistant cultivars.Consequently, we conducted trials to identify cotton cultivars that exhibit resistance to ACMoV.
A 3-year trial was conducted in northern Nigeria during the 1999-2001 wet seasons.The 25 long staple cotton cultivars obtained from the Fibre Research Programme, Institute for Agricultural Research (IAR) Samaru, Nigeria were used.The trial was laid out as a randomized complete block design and replicated three times.Each cultivar was allocated to a plot made up of two ridges, 4.5 × 1.2 m.Four seeds were sown per hole, 30 cm apart.The plots were separated from each other by a border row of 60 cm.Seedlings were thinned to two per stand, two weeks after germination.About 60 plants were grown per year for each of the 25 varieties.Ten ACMoV-infected cotton plants with a high population of B. tabaci were transplanted between adjacent cultivars to serve as source of inoculum.
The crop was fertilized at two and six weeks after germination with N.P.K. (20:10:10) at the rate of 20 kg ha -1 on each occasion.Disease incidence based on symptoms induced by ACMoV was recorded each year from one week after germination till the end of the growing season in December in order to obtain the percentage of infection.Disease severity was rated in individual plants using a visual scale of 1-7 where 1 = no visible symptoms; 3 = mild mosaic and slight leaf distortation; 5 = severe mosaic and leaf distortion; 7 = very severe mosaic, and severe leaf distortion.Resistance level was determined as outlined in Table 1.Cotton seed and lint harvested from each plot were weighed and recorded.
Each year data from percentage of infection and yield were analysed using the analysis of variance (SAS, 1988).Given that there were no differences among years, a joint analysis was done after which the standard error of difference (SED) between means was used to separate means of the different varieties that differed at 5% level of probability.
None of the 25 cultivars was immune or resistant.However two cultivars (Pima S 1 and Pima S 4 ) were moderately resistant.Four cultivars were moderately susceptible while 19 were highly susceptible (Table 2).There were significant differences (P ≤ 0.05) between the different disease severity classes (highly susceptible, moderately susceptible, and moderately resistant) for disease incidence and yield.Infected plants were mottled, stunted and reduced in size compared with healthy plants, which were uniformly green and showed no visible symptoms or malformations.Symptoms began about four weeks after germination.
There were similar results in all the three trials (Table 2).Pima S1 and Pima S4 were moderately resistant while Pima S2, Pima S3, Giza 45 and Giza 69 were moderately susceptible.The moderately resistant and moderately susceptible cultivars were also high yielding.This study reveals that there are two moderately resistant and four moderately susceptible cotton cultivars to ACMoV in northern Nigeria.Some cotton cultivars earlier identified as tolerant to ACMoV in some other African countries include EH9, Y1422 and HG9 (Cateland, 1971;Fournier and Cateland, 1971;Cateland and Bink, 1974;Cauquil and Follin, 1983;Malathi et al., 2003).As a result of the 1968-1971 epidemic of the disease in Chad (Malathi et al., 2003) the resistant variety HG9 replaced the susceptible BJA592, and the disease has been controlled since that time (Cauquil and Follin, 1983).Attempts made to obtain these cultivars for inclusion in this study was not successful.Hence the moderately resistant cultivars found in this study can be used to combat the menace caused by the disease on cotton in Nigeria.According to Cateland (1973), susceptibility to ACMoV originated from Triump, while resistance may be found in the N'Kourale lineage.Cultivars originating from the National Agricultural Research (NAR) triple hybrids (G.hirsutum × G. arboreum × G. raymondii are intermediate in susceptibility (Cauquil and Follin, 1983).The N'Kourale lineage could then be bred with the moderately resistant cultivars found in this study in order to obtain highly resistant cultivars.
The results obtained in this study were consistent during the 3-year probably because infector row infested with an efficient ACMoV vector, B. tabaci, were used between adjacent cultivars.This ensured that all plants had an equal chance of being infected.Also the trial was located in an environment with high population of ACMoV vector, thereby nullifying the chances of a plant escaping the disease.All the cultivars were colonized by whiteflies within 4 wk after germination.
The two moderately resistant cultivars were probably more resistant than others because they had an in-built resistant factor to the virus or because they were probably not preferred as hosts by the whitefly vector of ACMoV.The moderately resistant and moderately susceptible cultivars are long staple cotton with high quality lint characteristics such as crin percentage, bundle strength, and micronaire fineness (Alabi, 1999).All these characteristics are highly valued by the textile industries.Although the other 19 cultivars are highly susceptible, they are also high yielding, have good lint qualities and are long staple.The moderately resistant and moderately susceptible cultivars reported in this study could be used in combination with other control measures to combat the deleterious effect caused by this disease in Nigeria.The resultant effect will increase cotton yield and ultimately the farmers income.

Table 1 .
Determination of resistance level

Table 2 .
Performance of long staple cotton cultivars screened for resistance to African cotton mosaic virus (ACMoV) at Samaru in the 1999-2001 wet seasons HS: highly susceptible.MS: moderately susceptible.MR: moderately resistant.SED: standard error of difference.