Susceptibility of new potato varieties to the potato late blight oomycete Phytophthora infestans (Mont.) de Bary in Lithuania

Susceptibilidad de nuevas variedades de papa al oomiceto del tizón tardío Phytophthora infestans (Mont.) de Bary de la papa en Lituania

Rita Asakaviciute*, Almantas Razukas, Juozas Jundulas

Vokè branch of Lithuanian Institute of Agriculture. Zalioji a. 2, Traku Voke, LT–02232 Vilnius, Lithuania. *Author for correspondence: (rita.asakaviciute@voke.lzi.lt)

ABSTRACT

Potato late blight can completely eliminate this crop in developing countries. Therefore, the objetive of this research was to determine the susceptibility of several Lithuanian potato cultivars to late blight. The experimental design was a randomized split–plot, data were subjected to the ANOVA and the Tukey test was used to verify the significance of differences (p<0.05). Late blight infection in potato groups of various growing cycles lenght was different. The intensity of potato blight spread in various earliness potato cultivars depends not only on their earliness, but also on their biological–mathematical models of late blight prognosis.

Key words: Potato, cultivars, late blight, resistance, ecologic farms.

RESUMEN

El tizón tardío de la papa puede eliminar completamente este cultivo en países en desarrollo. Por tanto, el objetivo de este estudio fue determinar la susceptibilidad al tizón tardío de varias variedades cultivadas de papa en Lituania. El diseño experimental fue parcelas subdivididas al azar, los datos se analizaron con ANOVA y la prueba de Tukey se usó para verificar la significancia de las diferencias (p<0.05). La infección de tizón tardío en grupos de papa de distintos ciclos de crecimiento fue diferente. La intensidad de diseminación del tizón tardío en diversas variedades cultivadas depende no sólo de su precocidad, sino también de sus modelos biológicos/matemáticos de pronóstico de la enfermedad.

Palabras clave: Papa, variedades cultivadas, tizón tardío, resistencia, granjas ecológicas.

INTRODUCTION

Potato late blight, a disease caused by the oomycete Phytophthora infestans (Mont.) de Bary, is one of the world's most devastating plant diseases. Despite its historic significance, none of the potato cultivars currently grown in Europe have an adequate late blight resistance. Currently, late blight is responsible for annual multibillion Euro losses in both potato and tomato production. Furthermore, in developing countries, where funds for purchasing fungicides are limited, late blight can completely eliminate potato crops (Rubio–Covarrubias et al., 2005).

Resistance of plants to pathogens often depends on the activation of defense responses after pathogen infection. A key factor in this type of resistance is the perception of the pathogen by the host, which triggers the appropriate defense responses. When defense responses completely block pathogen development, the interaction between pathogen and plant is called incompatible. The pathogen genes that mediate the recognition and activation of host defense responses leading to incompatible interactions are called avirulence genes (Bisognin et al., 2005). Incompatible interactions are usually associated with a hypersensitive response in the host and a high degree of specificity between the pathogen and host genotypes.

P. infestans and potato cultivars interact according to the gene–for–gene model (Lee et al., 2001). These 11 R–genes suggest the presence of 11 corresponding virulence or avirulence factors in P. infestans, and genetic analyses on both host and pathogen have been performed to confirm the gene–for–gene model in this pathosystem (Song et al., 2003).

Agrometeorological conditions for the presence and spread late blight in Lithuania are present almost every potato growing season. The pathogen infects leaves, stems and later the whole plant. As a result of sexual reproduction oospores are formed, as it has been registered in many countries. Oospores can survive in the soil for about ten years. Local pathogen populations of the patogen become more aggressive, virulent, and the disease becomes more dangerous (Judelson and Blanco, 2005).

The genetic resistance of potato cultivars to P. infestans is one of the main achievements of potato breeding programs. Many wild potato species exhibit field resistance to P. infestans and they are of a high genetic importance for potato breeding (Kamoun, 2001). The biophysical mechanisms controlling late blight susceptibility in potato cultivars are complex and can involve differences in leaf and tuber cell defense biochemistry, as well as canopy structure, leaf anatomical variations and vine maturation rates (Kirk et al., 2001). P. infestans has mutable features, so it can overpass any resistance. When a limited potato gene bank is used in potato breeding for resistance to late blight programs, the disease can violate large potato cultivar groups in separate years. Potato cultivars that were described as resistant to P. infestans in Europe are severly injured today by late blight (Song et al., 2003).

A fast development of potato late blight under favourable conditions causes a massive spread disease, called epiphytoty. Disease spread depends on meteorologic conditions, the level of high infection and the potato growing period at the time of disease attack. Late blight epiphytoty may be represented in a diagram as a sigmoid curve. At the beginning of disease spread (bottom of S), the late blight development is low because of the low infection. Disease development reaches its peak when it covers 50% (middle part of S). It is the epiphytoty maximum. In the end of epiphytoty (in the diagram – top of S), disease development slows down because only small parts of plant tissues are not injured. In years of late blight depression, the spread diagram is partial, and in the of years mute disease development the diagram curve has a lengthy long shape. The shape of P. infestans disease development curves can also give information about potato cultivars' susceptibility (Hansen et al., 2005).

The objective of the present research was to examine tuber and foliage susceptibility of commercial potato cultivars to potato late blight on the bases of the development and spread of the disease in the potato field.

The evaluation was carried out at the Voke Branch of the Lithuanian Institute of Agriculture from 1992 to 2007. Potato cultivars were bred using the hybrid cross method. The cultivar VB Venta was obtained after crossing Priekulu visagrie×Pirmune s and VB Rasa was obtained from Cardinal×Viola. The cultivar VB Liepa resulted from No 34/36×Pirmune s. The cultivars VB Goda from Ausonia×Franzi and VB Aista from No 263×No 476–9.

Experimental plots were established on sandy loam on carbonaceous fluvial–glacial gravel eluviated soil (IDp) or Haplic Luvisols (LVh) according to the FAO–UNESCO classification.

Meteorological conditions are a very important factor for late blight disease. The weather conditions during the experimental years were diverse (Table 1).

General conditions of the trial

The trials were set up in conformity with the local agricultural potato growing practices. The dynamics of late blight spread was assessed every 7–10 d from the occurrence of the first disease symptoms until top killing. One hundred plants, 25 plants from every plot, were tested. The potato vine infection level was scored according to the attack: few plants with lesions, 1–2 lesions in a 10–m radius; 0.5–1–5 spots per plant; 1 – 5–10 spots per plant; 5 – about 50 spots per plant, or up to 1 in 10 leaflets lesions; 10 –about 10% of leaf area destroyed; up to 4 in 10 leaves destroyed; nearly every leaflet has lesions, plants still look normal; 25 – about 25% of leaf area destroyed; nearly every leaflet has lesions; plants remain normal; the field still looks green; 50 – about 50% of leaf destroyed; every plant with lesions; the field is still green but with brown spots; 75 – about 75% of leaf area destroyed; the field colour is between green and brown; 95 – only a few leaves left, but stems are still green; 100 – all leaves dead or dying. Late blight data obtained from the trial field were scored (percentage): late blight spread was assessed, and the first early potato cultivar VB Venta was used as control susceptible.

The experimental design was a randomized split–plot, data were subjected to the ANOVA and the Tukey test was used to verify the significance of differences (p<0.05).

RESULTS

In 2005, the year of P. infestans tests, potato cultivars' susceptibility was estimated by measuring late blight start, the disease progress and their spread dynamics. The first symptoms of late blight in the field were small, light to dark green, circular to irregular–shaped water–soaked spots. These lesions usually appear first on the lower leaves. Lesions often begin to develop near the leaf tips or edges, where dew is retained longest. During cool, moist weather, these lesions expand rapidly into large, dark brown or black lesions, often appearing greasy. The lesions are not limited by leaf veins, and as new infections occur and existing infections coalesce, entire leaves can become blighted and killed within just a few days. The lesions may expand down the petioles and stems of the plant.

The disease spreaded more intensively in earlier than in late maturity cultivars. Disease development was low until June 24^th (0.17–19.15%) when almost all cultivars had 100% infected plants. Within one week, disease intensity grew from 16 to 45%.

In 2005, the late blight disease reached the level of epiphytoty (Figure 1). The tested cultivars can be divided into groups according to disease development curves: 1) very susceptible – VB Venta and VB Liepa; 2) medium susceptible – VB Goda; 3) medium resistant – VB Rasa and VB Aista.

In 2006 at the end of June – beginning of July, conditions were favourable for late blight which was found in all of potato foliage: plant leaves, stems and the top of foliage. The rainy and warm weather of June and August was favorable for late blight.

Late blight fungus was first identified in VB Venta and VB Liepa potato cultivars on June 13^th and after one week in VB Goda. The first symptoms of the disease in the potato cultivar Vilnia were found on June 20^th and in Aista on June 30^th (Table 3).

In the middle of August, almost all foliage was dead in susceptible potato cultivars (VB Venta and VB Liepa), and in more resistant potato cultivars (VB Rasa and VB Aista) foliage injuries covered 31.2552.0%. By the end of August, without application of fungicides, they had 21.5–27.0% of green leaves. The infection speed (AIR) was lowest in VB Aista. The competitive area under the disease intensity curve (RAUDPC) was highes in VB Liepa cultivars (0.59) and the lowest in VB Aista cultivars (Table 3 and Figure 2).

In 2007 in the end of June – beginning of July, conditions were favorable for the late blight disease spread which was found in all of foliage, not only on leaves, but also on stems and the top of foliage. The rainy and warm weather of June and August was favorable for disease development.

The late blight oomycete was first found in VB Venta potato cultivars on June 13^th and one week later in VB Goda. First signs of the disease in VB Rasa were found on the 20^th and in VB Aista on the 30^th of June. In 2007 a late blight disease epiphytoty occurred. The diagrams representing late blight development are S–shaped curves. Until the beginning of August, the disease development was not intensive (0.75–45.0%), but late blight infection was present in 100% of plants (Table 4 and Figure 3).

Within one week, the disease developed rapidly and the injury intensity increased from 3.2 to 95%. By the middle of August, almost all foliage died in the susceptible potato cultivars VB Venta and VB Liepa (95.0 and 80.0%). In the more resistant VB Rasa and VB Aista potatoes, 8.25% and 3.2% of foliage were injured. At the end of August they had 46.25% and 86.25% of green leaves, even when fungicides were not applied. The lowest disease infection speed (AIR) was in VB Aista potato cultivars. The competitive area under the disease intensity curve was highest in the cultivars VB Venta (0.49), VB Goda (0.47) and VB Liepa (0.45) and the lowest in VB Aista (0.03).

Depending on the late blight disease development, the potato cultivars were divided into the following groups: 1) very susceptible (VB Venta and VB Liepa); 2) medium susceptible (VB Rasa and VB Goda); 3) medium resistant (VB Aista).

DISCUSSION

Late blight is caused by the oomycete Phytophthora infestans (Mont.) de Bary. Unlike most pathogenic fungi, the late blight oomycete cannot survive in soil or dead plant debris. For an epidemic to begin in an area, the oomycete must survive the winter in potato tubers (culls, volunteers), be reintroduced on seed potatoes or tomato transplants, or live spores must blow in with rainstorms. Disease development is favored by cool, moist weather. Under these conditions, lesions may appear on leaves within 3–5 d of infection, followed by white mold growth soon thereafter. Spores formed on the mold are spread readily by irrigation, rain and equipment. They are easily dislodged by wind and rain and can be blown into neighboring fields within 8–16 km or more, thus beginning another cycle of disease (Mukalazi et al., 2001; Haynes et al., 2004).

The newly developed potato varities are resistant to wart disease and the RO₁ patotype of nematodes. Other advantages, such as high resistance to fungal and bacterial diseases and viruses, are also given high priority in Lithuanian potato breeding.

Analyzing late blight development helps to determine disease development differences not only among potato cultivars and maturity groups of diferent levels of susceptibility, but also differences in the same potato cultivar in each year of research. Research on the influence of meteorological conditions on late blight development was performed in various potato maturity groups and susceptibility to disease in all vegetation periods.

Some scientists have stated that few methods are needed for evaluating potato cultivars susceptibility to late blight. In the future, potato cultivars and hybrids of potato breeding programs will have to be tested not only under field conditions with a natural late blight infection, but also at the laboratory under artificial infection settings (Asakaviciute et al., 2006; Razukas and Jundulas, 2005).

Potato blight development in various potato cultivars depended not only on their maturity group, but also on their genetic and biologic features. The most resistant to late blight were VB Rasa and VB Aista. According to susceptibility to late blight, potato cultivars were divided into three groups: very susceptible (B1), medium susceptible (B2) and medium resistance (B3).

Research on the potato cultivars of resistance to late blight is very important. In Europe, due to the fast evolution of P. infestans, it is possible to suggest that the resistance imported from countries can change, because the population of the late blight agent is different between countries. Resistance in imported new potato cultivars depends on how the racial constitution of the fungus P. infestans population influences the cultivars' susceptibility to late blight, and they should be always tested (Kirk et al., 2001).

Late maturity cultivars were more resistant to the disease. Potato late blight disease spread intensity in the different cultivars depended not only on their earliness, but also on their genotype.

CONCLUSIONS

In years of late blight disease epiphytoty pathogen agent P. infestans injured potato leaves, top of the plants, steams and tubers. It was observed in the following developmental stages: conidia was found on potato stems, top of the plants and leaves; mycelium – in leaves and tubers.

In our study most of the tested cultivars were medium susceptible and only the cultivar VB Aista was resistant to late blight. Thus VB Aista is recommended for growing on ecologic farms.

LITERATURE CITED

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