1. IntroductionPhurejas potatoes (Solanum phureja Juz et Buk) are grown in the Provinces of Larecaja, Camacho, Muñecas and Bautista Saavedra in the north of the Dept. of La Paz in the Bolivian Andes. These areas lie between altitudes of 3200 and 4000 m, their soils rich in organic matter. Their climates are strongly influenced by the La Paz Mountains and range from cold to temperate, although all have high humidity and suffer permanent cloudiness. Cárdenas [1] indicated phurejas potatoes to come from Ilabaya, close to Sorata, and from the Yungas highlands in the Chojlla region (highlands of the Yungas, Dept. of La Paz). These varieties are characterized by their early nature and culinary quality [2], and are very popular with consumers in urban markets. According to Hawkes and Hjerting [3], and to Ochoa [4], the greatest diversity of S.
phureja is found in the Dept. of La Paz.
The climatic conditions of the areas where these potatoes grow are favourable to the development of late blight (known locally as “Llejte”, an Aymarian native language word) caused by the endemic Oomycete Phytophthora infestans. However, farmers need to use very little fungicide or none at all, for acceptable yields of phurejas potatoes to be obtained. Under the same conditions, other native varieties such as S. tuberosum subsp. andigena var. Waych’a Paceña, and introduced varieties such as S. tuberosum var. Desiree, require intense fungicide treatment for acceptable production to be obtained. Cañizares and Forbes [2] indicate partial resistance to late blight to be one of the characteristics of phurejas potatoes. Indeed, certain varieties of S. phureja, along with varieties of S. tuberosum subsp. andigena other than var. Waych’a Paceña, are currently being used as genetic sources of durable resistance in potato breeding programmes [3,5]. Understanding the incidence and severity of late blight over the year is of great importance for its control and the development of disease management strategies [6]. The aim of the present study was to assess the resistance of phurejas potatoes to P. infestans in a traditional area of cultivation.
2. Materials and Methods2.1. Plant MaterialsThe experimental plants used in this work were five varieties of phurejas potatoes: 1) Polo Phureja (Solanum phureja-Sphu); 2) Phureja Roja (Solanum phureja-Sphu); 3) Chojllu Phureja (Solanum stenotonum-Sstn, a non Solanum phureja variety); 4) Phureja Negra (Solanum phureja-Sphu); 5) Phureja Amarilla (Solanum phureja-Sphu), plus 6) S. tuberosum subsp. andigena var. Waych’a Paceña (Sadg), a control susceptible to P. infestans. All the phurejas potatoes were collected in the locality of Chojchoni-Sorata, where they are a traditional crop and where P. infestans is endemic.
2.2. Field ExperimentAn experimental plot was set up in the area of Chojchoni (3200 m, on the Warisata-Sorata road) in the Province of Larecaja, Department of La Paz (15˚52'S, 68˚38'W, 120 km from the city of La Paz). The plot was planted following a completely randomised block design with six treatments and three replicates. The treatments were the five phurejas varieties mentioned above plus the susceptible control Waych’a Paceña (Figure 1). Cypermethin was used to control Premnotrypes and Epitrix. Ridomil MZ was applied to the plants at emergence to remove any naturally occurring P. infestans before the experimental infection of the plants. Chemical (80-120-00 kg/h) and organic fertilizer (Approx. 10 t/h) were applied at the time of planting, according to traditional practices. Tubers (N = 16) were planted in rows 5 m long, with 0.7 m between rows and 0.3 m between plants. The trial was performed in summer (November-April) during the peak time of P. infestans activity.
2.3. InoculumAfter emergence the plants were sprayed with 8 × 105 sporangia/ml of P. infestans collected from infected leaflets of Phureja Negra plants growing in the same locality.
2.4. Evaluation of Late Blight and Statistical AnalysisSeverity of infection was recorded on the Centro Internacional de la Papa (CIP) scale at 7-day intervals, beginning 101 days after planting, with 7 readings per treatment. A disease progress curve (DPC) was constructed according to Campbell and Madden [2]. The
area under the disease progress curve (AUDPC) was estimated as
, where n is the number of evaluations, yi is the disease severity, and t is the time (in days) post-inoculation [2]. Infection rates (r) were modelled according to Campbell and Madden [2], and the regression coefficients (R2) determined using the logistic model [ln(y/1 − y)] [6]. Yields were estimated as kg/plant and as t/h of healthy and P. infestans-rotted tubers.
3. Results3.1. Disease Progress Curve and Area under the Disease Progress CurveThe Waych’a Paceña plants showed a disease severity of 70% within 35 days, while the phurejas potatoes fell into two severity groups, one with scores similar to the control (Chojllu Phureja, Phureja Amarilla and Polo Phureja), and the other with much lower scores (Phureja Roja and Phureja Negra) (Figure 2(a)). The AUDPC for the phurejas potatoes also fell into two groups, one formed by the varieties Chojllu Phureja, Phureja Amarilla and Polo Phureja plus the control Waych’a Paceña, and the other formed by Phureja Roja and Phureja Negra (F0.05).
3.2. Disease Progress Curve: Linear Regression AnalysisThe linear regressions for the phurejas potatoes were different (Figure 3), though all returned high regression coefficients (R2 > 0.9). The varieties Chojllu Phureja (r = 0.074/day), Phureja Amarilla (r = 0.067 day) and Polo Phureja (r = 0.077 day) showed infection rates below that of the control (0.085 day). That of Phureja Negra (r = 0.079 day) was also smaller, while Phureja Roja showed an infection rate equal to that recorded for the control (r = 0.085 day) (Figure 3).
3.3. Yield (t·ha<sup>−1</sup>)Yields (t/ha) differed significantly between the different varieties (F0.01) (Figure 4(a)). The yield of Phureja Roja (7.5 t/ha) was significantly greater than those of all the other varieties. No significant difference was seen between the yields of Polo Phureja (6.38 t/ha) and Phureja Negra (6.3 t/ha), but both produced significantly higher yields than Phureja Amarilla (4.4 t/ha) and Chojllu Phureja (3.2 t/ha). In fact, the yields of these last two varieties were smaller than that recorded for the control variety (5.6 t/ha).
4. DiscussionThe resistance to late blight shown by phurejas potatoes
is currently being used in potato breeding programmes [2,3,5]. However the present work shows that different phurejas varieties have different levels of resistance. The phenological stage of the crop also seems to influence the development of the disease. In Chojllu Phureja, Phureja Amarilla and Polo Phureja, the disease began at the same time as in the Waych’a Paceña controls, but at 122 days after inoculation disease severity began to decline in these phureja varieties, as shown by their DPCs (Figure 3(a)). This determined a similar behaviour for the corresponding AUDPCs (Figure 3(b)) and probably explains the lower infection rates seen in these varieties than in the control (Figures 4(a)-(c)). Figures 3(a) and (b)
reveal the somewhat greater resistance of Phureja Roja and Phureja Negra, although towards the end of the experimental period disease severity appeared to increase.
In Bolivia, Waych’a Paceña is susceptible to late blight. In the traditional areas of seed potato production (Independencia, Morochata, Lope Mendoza and Colomi) in the Dept. of Cochabamba, and in other areas of the Depts. of Chuquisaca and Tarija, endemic late blight has to be controlled via eight or more application of fungicide. In contrast, phurejas potatoes are usually grown in their traditional areas without the need for such protecttion.
Linear regression analysis (logN disease severity against time) allows the infection rate during an epidemic to be determined [6-8]. The infection rates for Chojllu Phureja, Phureja Amarilla, Polo Phureja and Phureja Negra suggest these varieties possess partial resistance to late blight, while Phureja Roja appears to be as susceptible to the disease as the Waych’a Paceña controls.
The yield of the phurejas varieties was closely related to their level of resistance. Cañizares and Forbes [2] reported low AUDPC values and acceptable mean yields to indicate the possession of minor resistance genes. The high AUDPC values and low yields shown by Chojllu Phureja and Phureja Amarilla in the present work might therefore be a sign of their low copy numbers of late blight resistance genes. The level of tuber rot also varied from one phurejas variety to another, with Phureja Roja and Phureja Negra most affected, and Chojllu Phureja, Phureja Amarilla and Polo Phureja least affected.
In the study area (Sorata), farmers preferentially raise Phureja Negra and Phureja Roja potatoes since they provide them with greater yields. The cultivation of Chojllu Phureja, Phureja Amarilla and Polo Phureja is shrinking strongly; indeed it is quite uncommon for them to be planted. As the present results show, Chojllu Phureja is strongly affected by P. inefstans, thus it is among the varieties least preferred by farmers. In the mid-long term, artificial selection forced by late blight could lead to the disappearance of the most susceptible varieties.