Release of cowpea line IT99K-494-6 for yield and resistance to the parasitic weed, Alectra vogelii Benth. in Malawi

Field trials were conducted between 2008/09 and 2009/10 seasons to evaluate four new cowpea lines, selected in preliminary studies, for yield, adaptation and resistance to the parasitic weed Alectra vogelii in Malawi. The design was randomised complete block design with 4 replicates at research stations and 5 at on-farm sites. For on-farm, villages were the sites and each farmer hosted one replicate. The trial sites were Lisasadzi, Mngwangwa, Bunda College, Chitedze Research Station, Rivirivi and Mpokwa in 2008/09 season and in 2009/10 the sites at Chitedze and Mpokwa were replaced by Mkwinda and Chitala Research Station. Four promising cowpea lines, IT98K-503-1, IT97K-825-15, IT99K-7-21-2-2-1 and IT99K494-6, all originally from IITA and selected for resistance to parasitic weed Alectra vogelii in screen house studies, were compared with released varieties Sudan 1, IT82E-16 and a farmers local entry. The following parameters were evaluated: cowpea grain yield, plants/m, seeds/pod, seed weight (g/100 seeds), days to mid-flower, days to maturity, disease scores (scale 1 [clean] – 9 [most severe]) and A. vogelii emergence (plants/m). The line IT99K-494-6 was found to have comparable or better agronomic traits as compared to control varieties and was eventually released in Malawi in January 2011. The line which is medium maturing has brown grain texture with yield potential of 1-2.0 t/ha. Such a resistant variety is important in A. vogelii hot spots to reduce build up in soil seed bank and a viable option for farmers to rotate with cowpeas in order to manage other pests such as Striga asiatica, an important parasitic weed, or improve soil fertility through the nitrogen fixation of cowpeas.


INTRODUCTION
Cowpea is amongst the important food legumes in Malawi as it is drought tolerant and can do well in warm areas where beans cannot.In 2009/10 growing season, the yields of cowpeas under smallholder farmers in Malawi averaged 378 kg/ha from an area of 61,082 ha (MoAIWD, 2012).Cowpeas are an important source of protein averaging 22 -34% (Elias et al., 1964;Neil and Brits, 1992;Mahe et al., 1994) and cash for smallholder farmers.The main constraints to its production are insect pests, diseases and lack of suitable varieties (MoAFS, 2005).In 2009/10 growing season, most farmers planted improved cultivar Sudan 1, variety IT82E-16 and local seeds which are preserved and recycled.Potential yields of cowpea is 2,000 kg/ha (MoAFS, 2005).However, Alectra vogelii, a parasitic weed for legumes, is also of growing concern particularly as legumes have been widely promoted for some time (ICRISAT/MAI, 2000; Ngwira et al., 2012).The Malawi Government, through the Farm Input Subsidy Program (FISP) also promotes legumes (MoAIFS, 2007).The weed A. vogelii is widely seen in Lilongwe and Kasungu plains and many parts of southern region (Riches and Shaxson, 1993;Kabambe et al., 2005;Mainjeni, 1999;Kabambe et al., 2008a, b).In Tanzania yield losses of up to 50% have been reported (Mbwaga et al., 2000).In Kenya, Bagnall-Oakley et al. (1991) reported total crop loss in some parts.In Botswana, Riches (1989) reported losses of 80 -100% in a susceptible cultivar.Yield losses of up to 15% have been reported in groundnut in Nigeria (Salako, 1984), while in South Africa 30 -50% reductions in yield of bambara were reported (Beck, 1987).In northern Nigeria, Lagoke (1989) reported that late-sown crops of soybean may be completely destroyed by the parasite.The witchweed A. vogelii may be controlled through trap crops such as dolichos bean (Lablab purpureus [L.] Sweet) cotton (Gossypium hirsutum L.), okra (Hibiscus esculentus L.), sunflower (Helianthus annuus L.) pea (Pisum sativum L.) and faba bean (Vicia faba L.) (Parker and Riches, 1993).There is generally greater availability of resistant genotypes against A. vogelii (Riches, 2001;Mainjeni, 1999;Rubiales et al., 2006;Omoigui, 2012).Thus, production is threatened by A. vogelii and screening for resistance among existing legumes crops or varieties can help to avoid working with the most susceptible varieties.The parasitic weed Striga asiatica is also a serious problem in Malawi (Kabambe et al., 2002(Kabambe et al., , 2005)), such that the introduction of legume varieties with resistance to A. vogelii would be useful to encourage farmers to adopt legumes as trap crops for it or soil fertility replenishment.Also, resistant varieties would slow down the seed build-up of the pest in soils.This study was therefore conducted to evaluate some selected cowpea lines for yield adaptability and resistance to the parasitic weed A. vogelii.

METHODOLOGY Trials design, sites, test materials and management
Four cowpea lines, IT98K-503-1, IT97K-825-15, IT99K-7-21-2-2-1 and IT99K-494-6 were evaluated for yield and resistance to the parasitic weed species A. vogelii under field conditions in 2008/09 and 2009/10 in Malawi.These lines were ex-IITA (International Institute of Tropical Agriculture) and were selected for evaluation based on earlier screen house screening for resistance to A. vogelii and preliminary yield trials.The lines were compared to released varieties Sudan 1, IT82E-16 and farmers local entry.In both seasons, there were four mid-altitude sites and two low altitude sites.The names, geographic position and altitude of the sites are given in Table 1.The trial design was randomized complete block with four replications at station sites while on-farm there were five replicates, with one farmer hosting one replicate.Experimental plots had 5 with ridges each 4 m long and 0.75 m apart (gross plot size 15 m 2 ).No fertilizers were applied to plots.Two seeds of cowpeas were planted at 20 cm apart.Planting was done with first rains in low altitude sites and in mid-January for midaltitude sites according to standard recommendations (MoAIFS, 2005).There were 4-5 farmers per village or site, making one complete trial.

Data collection and analysis
All crop plant or A. vogelii data was collected from the three middle rows (net plot size 12 m 2 ).Data were collected for evaluation of plant cowpea grain yield (adjusted to 12.0% moisture), plant count/m 2 , A. vogelii counts/m 2 , seed weight, seeds/pod, pod length (cm) days mid-flower and maturity.Counts of A. vogelii were taken at time of flowering of cowpeas.Assessment for disease was done at Chitedze Research Station only, where the project pathologist is based.Entries were scored for Aschochyta blight (Ascochyta phaseolorum), Scab (Elsinoe phaseoli) and Cercospora leaf spot infection.Disease scores were based on 1-9 scale where 1 was clean and 9 was severe.For infection, the score was the percentage of plants infected.Data were analyzed on site basis according to the analysis of variance procedure using the Genstat package.Comparisons between means was done using the least significant difference, LSD, t α = 0.05.

Rainfall
In 2008/09 rainfall was normal and fairly distributed for all sites except Mngwangwa and Chitedze, which had relatively poorly rains in February 2009 (Table 2).

Days to mid-flower and maturity
The number of days to 50% flower in 2008/09 season is shown in Table 8.Plants flowered earliest at low altitude sites as compared to mid-altitude sites.The entry IT82E-16 was the earliest to flower at both mid and low-altitude sites (range 39-55 days, followed by Sudan 1 (range 41-59 days) then entries IT99K-494-6 and IT99K825-15 (range 49-57 days).In general, the maximum difference between flowering dates of the improved varieties was about 10 days.In 2009/10 the trend was similar in terms of variety responses.However, flowering was generally delayed by about 2 days (Table 9).The flowering time can vary depending on planting date which affects temperature exposure during the season.Discussions with farmers revealed that such differences were quite important to them.The effects of variety on days to maturity are shown in Tables 10 and 11.In 2008/09, plants matured early at Rivirivi and Mpokwa (66 days as compared to the other sites (76-81 days), which was expected as these sites are climatically warmer and drier areas.The entry IT99K-494-6 matured between 71 and 82 days in the midaltitude sites, as compared to 72-74 days for Sudan-1.In 2009/10 season, the entry IT99K-494-6 matured between 71 and74 days in the mid-altitude sites, as compared to 65 and 68 days for Sudan-1.

Disease incidence and scores
Assessment for disease was done at Chitedze Research Station only in 2008/09.Entries were evaluated for Aschochyta blight (Ascochyta phaseolorum), scab (Elsinoe phaseoli) and Cercospora leaf spot infection.The results showed no significant differences amongst     entries for early or late evaluation (Table 12).The scores for Aschochyta blight showed medium levels at both early and late stages of assessment while scab scores were quite high at the late scoring stage, averaging 6.0, which are close to severe.The infection of Cercospora and scores were also high.The high yields of the released varieties reflect a high degree of tolerance and adaptability.

Other agronomic traits, yield components and variety description
Trial means for other agronomic traits and yield components for 2009/10 season are shown in Table 13.All entries seemed to have similar plant establishment with a narrow range of 10-11.4plants per m 2 .The released varieties were generally superior on pod length and seeds per pod.The new lines were superior in terms of seed weights per pod.

DISCUSSION
The results are in agreement with other reports which show wide availability in resistant genotypes against A. vogelii (Mainjeni, 1999;Kabambe et al., 2008a, Mbwaga et al., 2000;Riches, 1989Riches, , 2001;;Rubiales et al., 2006;Omoigui, 2012).The results also generally showed that A. vogelii is naturally more prevalent in the mid-altitude sites where all entries are better adapted.The nature of these studies did not allow the researchers to determine yield loss due to A. vogelii parasitism, however, it is clear that the two local checks IT82E-16 and Sudan 1 were tolerant to it, owing to their high yields although they supported high numbers of A. vogelii.In Botswana, Riches (1989) reported losses of 80 -100% in a susceptible cowpea cultivar.In northern Nigeria, Lagoke (1989) reported that late-sown crops of soybean may be completely destroyed by the parasite.Although three of the new lines showed fair resistance (low support of Alectra), only the line ITK99K-494-6 was most consistent in yield at most sites.While yield is of interest to farmers, continued growing of susceptible cultivars promotes build-up of the seed bank.As A. vogelii is a parasite of other common legume crops in Malawi, such as groundnuts and soybean (Kabambe et al., 2008a), it is therefore sound management to reduce build up through planting varieties which support minimal Alectra, especially where yield is not compromised as is the case with IT99K-49-6.The A. vogelii may be managed through trap crops (false hosts) such as dolichos bean, cotton, okra, sunflower, pea and faba bean (Parker and Riches, 1993;Kabambe et al., 2008b).The same crops are trap crops for managing S. asiatica when grown in rotation or intercrops with cereals (Kabambe et al., 2005;Parker and Riches, 1993;Kabambe et al., 2002;Kabambe et al., 2008b;Parkinson et al., 1987;Carsky et al., 1994).By providing Alectra-resistant legume varieties to farmers, their choice to incorporate legumes in the farming systems improves.Legumes, including cowpeas furnish nitrogen through biological nitrogen fixation (Bado et al., 2006) and are important for integrated disease management.
The results of days to flower and maturity showed that the two of the new lines  were slightly late to flower and mature as compared to Sudan-1.There was no clear relationship between maturity period and yield observed.As an example, the variety IT82E-16 and IT99K-494-6 all gave highest yields at Lisasadzi in 2008/09 even though IT99K-494-6 matured at 71 days as compared to 60 days for IT82E-16.In the low altitude sites, however, IT82E-16 gave highest yields in both seasons.Earliness therefore was important in these short season areas.Results on diseases showed that no particular susceptibility or resistance for all entries such that existing pest and disease management practices would be recommended.Thus, the line IT99K-494-6 was recommended and released in January 2011 for the mid-altitude areas of Malawi due to consistence in resistance and high yield.

Table 3 .
Effect of genotype on Alectra counts m -2 taken after flowering at all the trial sites in 2008/09 season.
*Means in column denoted by same letter are not significantly different at 5% using LSD.

Table 4 .
Effect of genotype on Alectra counts m -2 taken after flowering at three sites in 2009/10.Means in column denoted by same letter are not significantly different at 5% using LSD. *

Table 5 .
Grain yield kg/ha of cowpea entries at mid-altitude trial sites in 2008/09.
*Means in column denoted by same letter are not significantly different at 5% LSD.
*Means in column denoted by same letter are not significantly different at 5% LSD.IT99K-494-6 gave the highest yields at Lisasadzi, Bunda and Mngwangwa, which were sites with highest Alectra pressure.In 2009/2010, the line IT99K-494-6 gave highest or joint highest yields at Lisasadzi, Mngwangwa

Table 7 .
Grain yield kg/ha of cowpea entries at two low altitude trial sites in 2008/09 and 2009/10 seasons.Means in column denoted by same letter are not significantly different at 5% LSD. *

Table 9 .
Effect of genotype on days to 50% flower in 2009/10.Means in column denoted by same letter are not significantly different at 5% LSD. *

Table 10 .
Effect of cowpea line on days to maturity in 2008/09 season.

Table 12 .
Summary of analysis of variance for disease scores at Chitedze Research Station, 2008/09.

Table 13 .
Trial means and standard deviation (in brackets) of yield components and other agronomic traits, 2009/10.