African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 215-223
El-Bedewy, O. M. Olanya, P. T. Ewell, C. Lung'Aho1, P. S. Ojiambo and J. Karinga
Code Number: CS01051
Resistance of potato clones of populations A and B to late blight (Phytophthora infestans (Mont) de Bary) was evaluated in field studies during 1999-2000 long
and short rains cropping seasons in Kenya at two locations, Kabete (1,800 masl)
and Loreto (2,400 masl). Treatments consisted of germplasm materials introduced
from International Potato Centre (CIP) headquarters in Lima, Peru from two populations
arranged in a completely randomised block design with three replications. At
Loreto, late blight was more severe during the long rains than in the short
rains while at Kabete late blight was severe during the short rains than in
the long rains. The most resistant late blight genotypes among the population
A at Loreto were clones 387193.21 and KP93743.6 with AUDPC values of 204.9 and
269.4, and yields of 50.6 and 46.9 t ha-1, respectively. These two clones had
also acceptable agronomic characteristics. Desiree (control) was the most susceptible
with AUDPC value of 2674.0 and a yield of only 9.8 t ha-1. The most resistant
genotype among the population B test materials at Loreto was clone 392640.28
with an AUDPC value of 252.9 and it had the highest yield of 53.4 t ha-1. Other
resistant genotypes were 392640.13 and 392640.7 with an AUDPC values of less
than 600 and also high yields of 41.8 and 40.5 t ha-1, respectively. The most
resistant clone at Kabete was 390013.10 with an AUDPC value of 37.6 while the
most susceptible one was 387969.1 with an AUDPC value of 671.9. Clone KP90182.4
had the highest yields of 35.0 t ha-1. Dutch Robjyn (susceptible check) had
also a relatively low AUDPC value of 491.3 with a yield of 27.6 t ha-1. Generally,
the low levels of disease observed at the two locations were due to low rainfall
and relative humidity during the growing seasons. Potato tuber moth (PTM) infestation
ranged between 0.5 to 20%. The high levels of PTM experienced were due to prolonged
drought during the growing season.
La résistance des clones de pomme de terre des populations A et B au mildiou a été évaluée dans les champs lors des saisons culturales de 1999-2000 à Kabete (1,800m) et à Loreto (3400m), au Kenya. Les traitements arrangés en blocs complètement randomisés avec 3 répétitions, consistaient en germoplasmes de deux populations reçues à partir du CIP-LIMA. A Loreto, le mildiou était plus sévère durant la petite saison des pluies que dans la grande. A Loreto, les génotypes plus résistants au mildiou issus de la population A étaient des clones 387193.21 suivis par KP93743.6 avec la valeur AUDPC de 204,9 et 269,4, et les rendements respectifs de 50,6 et 46,9 t ha-1. Ces deux clones avaient aussi des caractéristiques agronomiques acceptables. Désirée (contrôle) a été la plus susceptible avec une valeur AUDPC de 2674,0 et un rendement de 9,8 t ha-1. Le plus résistant clone de la population B à Loreto fut 392640.28 avec la valeur AUDPC de 252,9 et il a eu le plus grand rendement de 53,4 t ha-1 suivi par les clones 392640.13 et 392640.7 avec la valeur AUDPC inférieur à 600 et des rendements respectivement de 41,8 et 40,5 t ha-1. Le plus résistant clone à Kabete était 390013.10 avec une valeur AUDPC de 37,6 tandis que le plus sensible était 387969.1 avec une valeur AUDPC de 671,9. Le clone KP90182.4 a eu le rendement de 35 t ha-1. Dutch Robjyn (standard sensible) a aussi eu une valeur AUDPC relativement petite de 491,3 et le rendement de 27,6 t ha-1. Généralement les niveaux bas de la sévérité de l'infection observés aux deux endroits étaient dûs aux faiblesses de la pluviométrie et de l'humidité relative lors de croissance. L'infestation par des teignes des tubercules de pomme de terre (PTM) s'est rangée entre 0,5 et 20%. Les niveaux élevés de PTM notés, ont été probablement dûs à la sécheresse prolongée observée durant la saison culturale.
Mots Clés: Résistance horizontale, mildiou, teignes de la pomme de terre, rendement, PTM, Kenya, SSA, Centre International de la Patate (CIP)
Potato late blight caused by Phytophthora infestans (Mont.) de Bary, is a very devastating disease in the major potato (Solanum tuberosum L.) growing tropical highlands of sub-Saharan Africa. Serious economic consequences often result from complete or partial devastation of infected fields (Haverkort, 1986; Sengooba and Hakiza, 1997). Because of the rapid development of the disease, infections occurring during various stages represent enormous economic threat. With the exception of optimum or scheduled fungicide applications based on the favourable conditions, the most economically viable disease management option is the use of host-plant resistance. Some farmers attempt to control the disease with numerous fungicide applications, but this increases cost and creates uncertain environmental and health issues. In addition, due to the high cost of fungicides, many resource poor farmers are unable to suppress the disease adequately. Losses are heavy, and complete crop failure is not uncommon.
Because of the changes in late blight populations, optimum management of late
blight is often best accomplished by a combination of measures including the
use of host plant resistance (Umaerus et al., 1983). Various studies have been
conducted in order to determine the level of resistance to late blight available
in potato clones and varieties (Canizares and Forbes, 1995; Lung'aho et al.,
1998). Other researchers have documented varietal differences in resistance
to late blight in foliage and tubers (Inglis et al., 1996; Niemira et al., 1999).
Development of potato varieties with resistance to late blight is an important
component in the management of potato late blight. Because of differences in
environmental conditions and variability in potato varieties or clones for various
use (fresh market, processing, crisps and chips), evaluation and screening of
varieties for resistance to late blight continues to be an important component
in the Integrated Disease Management (IDM) of late blight (Milgroom and Fry,
1988). The objective of our study was to assess the resistance and susceptibility
of potato clones from population A and B for late blight infection under field
conditions and to identify resistant lines in Kenya. Resistant clones and varieties
from populations A and B with acceptable agronomic characteristics will be selected
and cleaned - up for distribution to various National Agricultural Research
Systems (NARS) in Sub-Sahara Africa.
Field plot establishment. Field plots were established at the medium and high altitude locations at the University of Nairobi Experimental Farm at Kabete (1,800 masl) and Loreto (2,400 masl) in 1999-2000 long and short rain seasons. Experiments were planted on 2 April 1999 and 19 October 1999 during the long and short rain seasons, respectively. In the two seasons, the experiment consisted of twenty hill plots, arranged in a randomised complete block design with three replications. The number of clones from population A and B planted in 1999 at both locations were 47 and 43, respectively. Both populations A and B were evaluated at Loreto while only population A genotypes were tested at Kabete. In all experiments, susceptible varieties, Dutch Robjyn and Desiree were used as susceptible checks with Tigoni as the resistant check. All agronomic and crop management practices for potato production were followed at the two locations. Selection of potato genotypes was based on their resistance to late blight, high yield and acceptable agronomic characteristics (skin colour, tuber size and shape, eye depth, plant vigour and maturity).
Disease assessment. In both seasons of clonal evaluation, no artificial inoculations were conducted in the test plots. Late blight disease was initiated from natural infections as a result of disease spread from nearby fields. No fungicide applications or disease control practices were conducted. Foliar disease severity was visually assessed as a percentage of foliage blighted. This was done weekly starting 40 days after planting to give a total of 6 weekly late blight readings. Assessment of plant vigour and maturity parameters was also conducted in all experimental plots. At maturity, vine removal was conducted by de-haulming. Depending on the environmental conditions dehaulming was carried out 1-2 weeks before harvesting to allow for adequate curing of the tubers. Potato tubers were harvested per plot and weighed and yield converted to t ha-1 for each clone. Tuber numbers and tuber sizes were also recorded for each treatment plot. To determine tuber blight infection, a sample of suspected tuber blight infected tubers were taken to the laboratory. Tubers were then washed, and the number of blighted (diseased) and non-blighted (non-diseased) tubers were determined by a visual observation of tubers with tuber blight symptoms, confirmed by laboratory incubation and induction of fungal sporulation.
Data analysis. To determine the differences in resistance and susceptibility of potato clones to late blight, disease severity, tuber blight, and yield data were subjected to analysis of variance of the statistical analysis system. To determine the ranking of potato clones to late blight infection, data were subjected to mean and rank analysis of statistical analysis system. The relationships of disease severity and tuber blight in fields were also computed by using correlation analysis of the statistical analysis system (SAS, 1988).
Loreto (2,400 masl). Germplasm from both populations A and B were evaluated at this location. Late blight disease severity was measured as area under disease progress curve (AUDPC) and among population A genotypes, the most resistant clone was 387193.21 followed by KP93743.6; they had AUDPC values of 204.1 and 269.4, respectively. Disease was most severe on clone 390381.32 with an AUDPC value of 1437.3. Other highly susceptible clones were KP90114.7 and 392165.62 with AUDPC values of 1087.4 and 1120.8, respectively. Significant differences in disease reaction among the clones were also observed at this location. All the clones evaluated during this season had low disease severity compared to the susceptible check Desiree which had an AUDPC value of 2674.0 (Table 1). Over 60% of the clones evaluated were less susceptible to late blight compared to the tolerant check, Tigoni. Tuber yields during this long rain season ranged from 9.8 t ha-1 for the most susceptible check, Desiree, to as high as 50.6 t ha-1 for the most resistant one KP93743.6. Significant differences in tuber yield among clones were observed during this season. Approximately 50% of these clones yielded higher than the tolerant check, Tigoni. Potato tuber moth (PTM) infestation during the long rains were minimal ranging from 3.5% to 16% (Table 1) and no significant differences in PTM infestation were observed among the test clones.
Disease severity levels during the short rain season were significantly lower than disease levels experienced during the long rains. AUDPC values ranged from as lower as 87.5 for clone 387967.2 to 654.7 for the susceptible check variety, Dutch Robjyn (Table 2). Although disease levels were significantly different among some clones, the level of significance was very minimal. Other genotypes that had low disease levels included 392165.62 and 387193.21 (Table 2). All the clones evaluated had low disease levels than the check variety Asante. The highest yielding clone was 387193.21 that had a yield of 49.9 t ha-1 and was significantly different from that of other clones except clone KP90154.1 with yield of 42.4 t ha-1 and KP93743.6 with a yield of 45.27 t ha-1. Approximately 70% of the clones evaluated during this rainy season yielded higher than the control checks Asante and Rutuku (Table 2). Compared to the long rain season, the PTM infestation levels were lower during the short rains. Infestation levels were low and ranged from 0.67% to 10%, with clone 390381.32 being the most highly infested genotype followed by clones 387967.2 and 390012.2. The level of infestation on these three clones was significantly higher than on all other genotypes except clones 392145.44, 392165.62 and KP93743.6 that had similar levels of infestation and tuber damage.
Among the population B genotypes evaluated at Loreto during the long rains, clone 392640.28 was the most resistant with an AUDPC of 252.9 followed by 392627.13 with an AUDPC value of 307.1. Clone 392640.28 followed by clone 392640.13 had about 50% seed sized tubers from the total number of tubers harvested. The most susceptible genotypes were 391049.15, 392627.6, 391058.56 and 392622.12 that had AUDPC values of 1034.3, 1077.1, 1105.1 and 1481.4, respectively (Table 3). Compared to population A clones evaluated in the long rains, population B genotypes exhibited low disease levels. Generally, these sets of genotypes also had higher yields than population A genotypes, with yields ranging from 53.4 t ha-1 for 392640.28 to 21.4 t ha-1 for clone 391049.89. Other high yielding clones were 392640.13, 392640.7 and 391049.77 (Table 3). Highly significant differences in yield were also observed among the population B genotypes.
Kabete (1,800 masl). Reaction of potato clones from population A to late blight infection at Kabete is summarised in Table 4. Disease was very low during the long rains than in the short rain season due to unexpected low rain and relative humidity that resulted in unfavourable conditions for disease progression. Therefore, disease severity levels were only recorded in the short rains. AUDPC values ranged from as lower as 37.6 for clone 390013.10 to only 671.9 for clone 387969.1 (Table 5). Yields ranged from 47.5 t ha-1 for KP90182.4 to 22.0 t ha-1 for clone KP92472.1. Over 80% of the selected genotypes yielded higher than the tolerant check Tigoni which had a yield of only 28.6 t ha-1 (Table 4). Yields were significantly lower during the short rains than those observed in the long rains . Yields ranged from 15.6 t ha-1 for clone 387969.1 to 35.0 t ha-1 for KP90182.4. Only 3 genotypes (800946, KP90182.4 and 390013.10) had higher yields than those of the newly released varieties Asante and Tigoni (Table 5). Tuber blight during this season ranged from 0.4% for Tigoni to only 1.3% for clone KP93751.18 with a mean tuber blight infection of only 0.4% (Table 5). No significant differences in tuber blight infection were observed among the genotypes during this season. High infestations of PTM were observed during the short rain season and infestation levels were as high as 20% for KP92681.8.
Evaluation of genotypes from population A for late blight resistance have been previously documented elsewhere (Landeo et al., 1996; Landeo et al., 1997). In this study conducted in Kenya, variability in the reaction of potato clones from population A was recorded during the short and long rain season of 1998 - 1999 at both Loreto and Kabete locations. Out of the 47 clones evaluated at Loreto, 4 can be classified as highly resistant (AUDPC of less than 500), and 11 (AUDPC of 600-900) as moderately resistant to late blight. Among the clones evaluated at Kabete, none of them can be categorised as resistant to late blight due to low late blight pressure at Kabete during that season. This was due to lack of rain that resulted in unfavourable conditions for disease progression. Variability in late blight pressure among locations and over seasons has been previously reported. The lack of significant correlation of late blight severity with PTM infestation and clonal maturity groups suggest that the traits were not significantly correlated. The clones classified as late blight resistant had also better yield at this location. Results from previous work showed that genotypes from population A have high levels of resistance to late blight (PRAPACE/CIP, 1996; Lung'aho et al., 1997). However, disease levels varied among the selected genotypes and released varieties depending on the inoculum levels and environmental conditions. Interactions between genotype and environment has also been previously reported (Haynes et al., 1998).
A comparison of environmental parameters for Loreto (2,400 masl) and Kabete (1,800 masl) in the long rain season of 1999, indicate that conditions were more conducive for disease development at Loreto. In contrast, more late blight was detected at Kabete than at Loreto during the short rains. During 1999/2000, average rainfall amounts and temperatures during the short rain season were slightly more conducive at Kabete than at Loreto. Supplementary irrigation was also applied at Kabete during the short rains. This slightly improved the environmental conditions making it suitable for late blight progression for a short period of time.
A number of potato genotypes with quantitative resistance to late blight lack sufficient resistance to avoid or even reduce the use of fungicides. However, such germplasm may be used to initiate breeding programmes targeted at increasing the level of resistance in agronomically acceptable clones and cultivars. This explains why the International Potato Centre (CIP) has undertaken research in the development of population B. This population that segregates for quantitative resistance to late blight is composed of genotypes that express horizontal resistance (Anon., 1995). In this study, the selected clones from population B such as 392640.28 were more resistant to late blight than clones from population A and varieties at Loreto. Similarly, yield levels and agronomic traits were also significantly better among these clones than the clones from population A and released varieties. When comparisons were made between clones from population A and B, higher susceptibility was noted on early maturing clones. Therefore, early maturing clones or varieties could be more susceptible to late blight than late maturing clones. Varietal susceptibility as a function of maturity class is similar to those previously reported (Fry and Apple, 1986). However, further studies are needed to determine the relationship of earliness and resistance to late blight among population B genotypes. The ranking of foliar late blight severity at times differed from tuber blight severity. Clones such as 387967.2 and 390012.2 had very low AUDPC but greater tuber blight infection. Therefore, tuber blight infection may be impacted by numerous factors such as inoculum amount, varietal susceptibility, soil factors and management practices. Differences in foliar and tuber reactions to late blight have previously been reported (Olanya et al., 1999).
At the two testing sites, infestations of potato tuber moth (PTM) and potato viruses were observed. Although differences in PTM infestations were observed among the potato clones, no significant differences between the clones from populations A and B were observed. Variability in virus incidence was also observed on the clones from both populations. Therefore, precautions should be exercised in the evaluation of clones in the presence of secondary pests that are seasonal and may cause yield instability under certain conditions.
Although the Population B genotypes are showing great potential in terms of yield, agronomic traits and resistance to late blight, there is a need to determine the durability and stability of their resistance to late blight since they are in an early stage of evaluation. Further studies are also required to determine the relationship of earliness and resistance to late blight among population B genotypes. In addition, the advanced SIFT (Standardized International Field Trials) and SETS (Standardised Evaluation Trials) germplasm will provide further information on the sources of resistance to late blight of potato. These sets of materials (SIFT and SETS) are currently under massive propagation at CIP-KEPHIS facilities in Kenya. The relationship between PTM infestation and environment needs to be studied further to determine their interaction. Other diseases such as bacterial wilt (BW) and viral resistance need to be investigated in the selected and promising genotypes across locations.
TABLE 1. Performance of Population A clones during 1999 long rain growing season at Loreto, Kenya
AUDPC = Area under disease progress curve
TABLE 2. Performance of Population A clones at Loreto, Kenya during 1999 - 2000 short rain season
TABLE 3. Performance of Population B clones during 1999 long rain cropping season at Loreto, Kenya
1AUDPC = Area under disease progress curve
2Percentage of seed tubers
TABLE 4. Performance of Population A clones during 1999 long rain season at Kabete, Kenya
1Percentage of seed tubers
TABLE 5. Performance of Population A clones during 1999 - 2000 short rains season at Kabete, Kenya