Introduction

Potatoes are a major food crop in Southwest (SW) China, which includes over 40% of the total potato area in China (Jansky et al. 2009). Potatoes are grown year round in this area, but primarily as a summer crop from April to September in the highlands (over 2,000 m above sea level). At lower elevations, the potato is grown as an autumn crop (July to December), a winter crop (October to March), or an early-spring crop (December to May). Late blight caused by P. infestans is the single most important production constraint, particularly during the wet summer months (Song and Wang 1993).

The cultivar Mira, which originated from Germany, had been the dominant cultivar since the 1960s, occupying about 80% of the total potato production area in SW China until the mid-1990s (Wang et al. 1989). Although this cultivar has very good adaptation to this area and is mainly grown in the highlands, its productivity has declined due to virus infection and susceptibility to the changing population of P. infestans. Substantial improvement in potato productivity would be attained by identifying cultivars with excellent resistance to viruses and P. infestans, and by having an adequate basic seed-tuber production program in the sub-tropical highland areas of Yunnan.

In 1986, the International Potato Center (CIP) and the Yunnan Normal University (YNU) identified a common goal: to select superior clones for late blight resistance, eating quality, and virus resistance, along with initiating a tissue-culture-based basic seed production program. This would rapidly multiply promising clones for distribution and further multiply advanced clones or newly released cultivars. Thus, a number of cultivars and advanced clones were introduced from CIP and evaluated under the local conditions. Four cultivars (or clones), namely I-1085 (Sita), CIP-24 (B71.240.2, also known as Achirana Inta), CIP 378711.7 and CFK69.1, were selected from the CIP germplasm, due to their high level of resistance to P. infestans and yields above the check cultivar Mira (Wang et al. 1989). These clones were desirable as parental lines in the researchers’ efforts to develop superior adapted clones with P. infestans resistance.

Strategy

Late blight resistance is the key production constraint in all of the sub-tropical highlands. Generally, the material with late blight resistance has been poorly adapted to the longer day growing conditions in the highlands. A CIP-supported MSc. thesis breeding project was initiated in the Philippine highlands with a Vietnamese graduate student (Chien 1989). The strategy was to choose the better adapted late blight resistant CIP clones as the female parents and then utilize the CIP germplasm that is very late blight resistant but poorly adapted as a source of bulk pollen. The thesis involved two cycles of recurrent selection.

Extra true potato seed (TPS) from this thesis project was provided to the Yunnan project for evaluation. Over 8,000 seeds from 17 crosses were initially evaluated in 1990 at the Agricultural Extension Station in Huize county, which is over 2,200 m above sea level. After nine rounds of selection and trials in 5 years, only a few clones remained, with selection #88 being the most promising one for release (He and Wang 1993). In 1995, #88 passed all multi-locational and provincial trials. In 1996, at a special meeting in YNU, the selection was released as a cultivar for Yunnan Province and named Cooperation-88 in recognition of the collaborative work that resulted in this cultivar.

In 1994, after YNU did meristem culture to eliminate viruses, they entered the cultivar into the basic seed program. The first batch of mini-tubers was produced in 1995. Evidence from all the field observations indicated that C88 was an outstanding clone with strong demand from farmers and consumers, so production of mini-tubers continued.

Description

Pedigree

The female parent, I-1085, is a white-skinned, white flesh potato clone with late maturity, high yield and chipping quality (Wang et al. 1989). It is a selection from the India potato program which was sent to Mexico for LB screening. CIP obtained it for distribution and utilization. I-1085 was introduced to YNU in 1986, released in 1991, and gained popularity with growers in Yunnan, soon reaching over 10,000 ha in production. It was also a promising clone in the Philippines.

The male parent is a bulk pollen source from a population of late blight resistant clones selected in the northern highlands of the Philippines in 1986 from late blight resistant germplasm obtained from CIP Lima, Peru. These clones were considered to be poorly adapted to longer day conditions at 20o to 30o latitude. The pollen was collected from all fertile flowers, mixed, and then used to pollinate.

Foliage

The plants are vigorous with an erect growth habit. During the long days of summer, there is an abundance of flowers and berries (Fig. 1). The crop matures in 120–150 days during the long days of summer with up to 13.5 h of daylight, but during the winter season the crop canopy is smaller and matures in 80–120 days with virtually no flowering.

Fig. 1
figure 1

C88 field in the summer season in the highlands of Yunnan Province

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Tubers

Tubers are generally large and blocky (Fig. 2). Tuber skin is reddish pink with more pigmentation around the shallow eyes. The flesh colour is a light yellow, similar to Yukon Gold. The tubers are moderately resistant to bruising and to greening in the light. Consumers are paying a substantial premium for C88 as table potatoes, compared to the cultivar Mira.

Fig. 2
figure 2

Tubers and tuber flesh of C88

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Late Blight Resistance

Since the selection of C88 in 1995, it continues to show a high level of durable resistance to foliar P. infestans infection, and it is highly resistant to tuber infection. Late blight resistance for C88 has been evaluated annually from 2002–2009 under local conditions, where all detectable virulence genes existed naturally every year. The inoculation test with a set of isolates included IPO 428.2 (A2 mating type, 1.2.3.4.5.6.7.8.9.10.11) and X-4 (A1 mating type, 1.2.3.4.5.6.7.8.9.10.11), further confirmed its resistance to P. infestans compared with very susceptible cultivar Bintje and susceptible cultivars Atlantic and Mira. For C88, small and restricted lesions were noted with few mycelium. Its durable resistance is attributed to its S. andigena background, which carries R1 and R3a genes/analog (Li et al. 2008), which have been cloned and sequenced (Ballvora et al. 2002; Huang et al. 2005). In August, when the summer crop matures, late blight is present and is washed into the soil by the rain. Even when the farmers delay the harvest until December, no tuber infection is noted.

Virus Resistance

C88 is highly resistant to PVYo and PVX, and moderately resistant to PLRV when inoculated in the greenhouse. This permits farmers to keep their seed for many generations. One can observe many fields of C88 before finding a single visually infected virus plant, even though other cultivars are heavily infected with PLRV and PVY.

Nematode and Scab Resistance Under Field Conditions

C88 is moderately resistant to Globodera pallida race 1 and 2. It has low resistance to G. rostochiensis race 1. Resistance to common scab (Streptomyces scabies) is medium, similar to the cv. Atlantic.

Agronomic Performance

C88 performed well in all the comparative farm trials conducted (Table 1). In Huize, where C88 was initially selected, it produced excellent yields of over 50 t/ha using the seed tubers from greenhouse grown stem cuttings in 1994 and 1995 (Table 1). In subsequent trials, farmer-harvested and stored tubers were retained for seed. Although yield did decline, it remained above 30 t/ha, averaged over a large number of on-farm trials (Table 1). The decline in yield is primarily due to the physiological ageing of farmers’ seed, which is generally stored under sub-optimal conditions. The possibility of virus infection, however, particularly due to PLRV, could also be a contributing factor to the observed yield decline.

Table 1 Yield of C88 grown in plots of at least 0.07 ha in fields with the CV Mira (local check) in Huize County, Yunnan province
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In Xundian County (2400 m. asl), the C88 started from clean seed in 1996. Many farm trials were done with C88, averaging over 35.4 t/ha and yielding 37 to 68% more than Mira. These farm trials clearly reflect why C88 has rapidly gained its popularity in Yunnan province (Table 2). It is now ranked as the most widely grown cultivar in the Province. At first, the summer crop was the most popular season; however, C88 is now preferred during the short day winter season as well, yielding over 30 t/ha in 80 to 90 days. The lower elevation farmers obtain C88 seed from highland summer crop growers.

Table 2 Increasing popularity of C88 in Yunnan Province
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Utilization

The predominant use of C88 is for fresh consumption. Farmers get a premium price because consumers prefer the appearance and taste of C88. It can be compared to Yukon Gold for shape, flesh colour, cooking consistency and taste. C88 is also sold for table potatoes to neighboring countries, especially Vietnam during its rainy season from July to November.

Furthermore, C88 is now the preferred cultivar for the potato chip industry in Yunnan. Attempts to grow the cultivar Atlantic failed due to late blight presence and poor seed quality. C88 can be utilized throughout the year for producing potato chips. Freshly harvested potatoes are available from February to September. For the October to January period, the farmers agree to delay harvesting the potatoes destined for chips until they are needed. In early February, they transition from delayed harvest potatoes to fresh harvest (Fig. 3). For the delayed harvest, the crop stays in the ground until December, when soil temperatures can drop to 6°C. The crop is harvested by late December and stored for up to 1 month at the warehouses of middlemen or at the processing factory. The chip plants only utilize the medium-sized tubers. The larger tubers are utilized for the fresh market, and the small ones are often kept for seed.

Fig. 3
figure 3

C88 chips from a winter crop fresh harvest and a summer crop delayed harvest

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Production Trends and Spread of C88

The area under production with C88 has also spread to the neighboring provinces of Guizhou (96,670 ha), Sichuan (60,000 ha), Guangxi (30,000 ha) and Chongqing (16,670 ha), totaling 203,340 ha in 2009, which surpasses that of Yunnan (Table 3). Seed of C88 is now also spreading to Vietnam and Myanmar. The rapid spread of C88 is mainly attributed to its high yield, good culinary and chip quality, as well as for its high resistance to viruses and late blight. These factors have promoted its rapid adoption in the sub-tropical highlands because farmers can retain the seed with good health by themselves for more than one decade.

Table 3 Area planted to C88 and its average yield in neighboring provinces in 2009
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In comparison to C88’s production area of 390,000 ha, the leading cultivar in the USA and Canada, Russet Burbank, occupies about 175,000 ha. In India, Kufri Jyoti and Kufri Bahar are the two leading cultivars, with 650,000 and 480,000 ha respectively (Mohinder Kadian personal communication). There are also several cultivars that cover a larger area in the northern part of China.

Clones from the same MSc. Thesis (Chien 1989) are now also the major cultivars grown in the Philippines highlands, as well as in the southern highlands of Vietnam, especially the clone initially named LBR 1–5 (Chien 1989), and later named PO-3 in Vietnam and Igorota in the Philippines. As C88 continues to increase in importance, the huge success of this thesis project—particularly as it has benefited poor highland potato farmers—would be an important socio-economic study.