Relationship between late blight [Phytophthora infestans] of potato on ...

372

Epidemiology / Épidémiologie

Relationship between late blight [Phytophthora infestans] of potato on tuber and foliage, as affected by the disease severity on foliage, cultivar resistance, and atmospheric and soil variables R.O. Nyankanga, H.C. Wien, O.M. Olanya, and P.S. Ojiambo

Abstract: Potato tuber blight, caused by Phytophthora infestans, is an important component of the late blight pathosystem. Although the dynamics of tuber blight on potato (Solanum tuberosum) cultivars have been evaluated, the effects of climatic and soil variables on tuber blight incidence have not been adequately quantified. Field experiments were conducted at two distinct environments: in New York (1998–1999) using the A2 mating type (US-8 clonal lineage), and in Kenya (2000–2001) using the A1 mating type (US-1 clonal lineage). Fungicide applications were scheduled to vary the amount of blight infection. Foliar and tuber blight development, climatic variables, and soil variables were quantified during the cropping seasons. Fungicide application did not have a significant effect on the incidence of tuber blight at both locations. Soil temperature, precipitation, tuber depth, and cultivar resistance were significantly correlated with incidence of tuber blight at both locations. At Freeville, New York, precipitation when soil temperature was 16–18 °C had the highest correlation (r2 = 0.632) with tuber blight, whereas soil moisture had the highest correlation (r2 = 0.577) with tuber blight in Kenya. Path coefficient analysis showed that total precipitation during the epidemics and days when soil temperature was 16–18 °C had the largest direct effect on tuber blight in New York and at the field sites in Kenya. Regression models using atmospheric variables, soil variables, and cultivar resistance had moderate predictive ability of tuber blight at New York (0.44 < R2 < 0.61) but low prediction in Kenya (0.40 < R2 < 0.46). Similarly, cultivar specific models using foliar blight, atmospheric variables, and soil variables resulted in significant predictions of tuber blight in New York (R2 > 0.46), whereas few regression equations for Kenya resulted in significant prediction of tuber blight. These results suggest that cultivar resistance, soil variables, and atmospheric variables are the main determinants of foliar and tuber blight infection when inoculum is present. Key words: Phytophthora infestans, Solanum tuberosum, tuber blight, cultivars, environmental factors, resistance. Nyankanga et al.: potato late blight / tuber blight / effect of environment / cultivar resistance 387 Résumé : La brûlure du tubercule de la pomme de terre, causée par le Phytophthora infestans, est une importante composante du pathosystème du mildiou. Quoique la dynamique de la brûlure du tubercule de la pomme de terre (Solanum tuberosum) sur les cultivars ait été étudiée, les effets des variables climat et sol sur l’incidence de la brûlure du tubercule n’ont pas été suffisamment quantifiés. Des essais sur le terrain furent menés dans deux environnements distincts : dans l’état de New York (1998–1999) avec le type sexuel A2 (lignage clonal US-8) et au Kenya (2000–2001) avec le type sexuel A1 (lignage clonal US-1). Des applications de fongicides furent programmées de façon à faire varier la quantité de brûlure. Les variables développement de la brûlure des feuilles et du tubercule, climat et sol furent quantifiées pendant les saisons de végétation. L’application de fongicides n’a pas eu d’effets significatifs sur l’incidence de la brûlure du tubercule aux deux endroits. La température du sol, les précipitations, la profondeur des tubercules et la résistance des cultivars étaient significativement corrélées avec l’incidence de la brûlure du tubercule aux deux endroits. À Freeville, New York, les précipitations lorsque la température du sol était située entre 16 et 18 °C furent les plus fortement corrélées (r2 = 0,632) avec la brûlure du tubercule, alors que l’humidité du sol fut la plus fortement corrélée (r2 = 0,577) avec la brûlure du tubercule au Kenya. L’analyse des

Accepted 11 October 2007. R.O. Nyankanga1 and H.C. Wien. Cornell University, 134A Plant Science, Ithaca, NY 14853, USA. O.M. Olanya.2 USDA Agricultural Research Service, New England Plant, Soil and Water Laboratory, Orono, ME 04469, USA. P.S. Ojiambo. International Institute of Tropical Agriculture, PMB 5320, Ibadan, Nigeria. 1 2

Present address: Department of Plant Science and Crop Protection, University of Nairobi, P.O Box 30197, Nairobi, Kenya. Corresponding author (e-mail: [email protected]).

Can. J. Plant Pathol. 29: 372–387 (2007)

Nyankanga et al.: potato late blight / tuber blight / effect of environment / cultivar resistance

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coefficients de direction montra que les précipitations totales pendant les épidémies et pendant les jours où les températures du sol étaient de 16 à 18 °C eurent le plus gros effet direct sur la brûlure du tubercule aux lieux des essais de l’état de New York et du Kenya. Des modèles de régression basés sur des variables atmosphériques et telluriques et sur la résistance variétale furent moyennement capables de prédire la brûlure du tubercule dans l’état de New York (0,44 < R2 < 0,61), mais ne le furent que faiblement au Kenya (0,40 < R2 < 0,46). De même, des modèles de régression spécifiques au cultivar et basés sur les variables brûlure des feuilles, atmosphère et sol ont généré des prédictions significatives de la brûlure du tubercule dans l’état de New York (R2 > 0,46), alors qu’au Kenya, peu d’équations de régression ont donné des prédictions significatives de la brûlure du tubercule. Ces résultats suggèrent que les variables résistance variétale, sol et atmosphère sont les principaux déterminants de l’infection des feuilles et du tubercule lorsque de l’inoculum est présent. Mots-clés : Phytophthora infestans, Solanum tuberosum, brûlure du tubercule, cultivars, facteurs environnementaux, résistance.

Introduction Potato late blight, caused by the oomycete Phytophthora infestans (Mont.) de Bary, is the most devastating disease of potato (Solanum tuberosum L.) worldwide (Erwin and Ribeiro 1996). The disease affects the foliage and tubers, causing yield loss as a result of death of the haulm and tuber rot in the field and in storage. Blighted tubers are unsuitable for human consumption or seed and are very susceptible to secondary soft rots. Survival in potato tubers provides an overwintering mechanism for P. infestans (Andrivon 1995), and infected tubers can be important sources of initial inoculum (Hirst and Stedman 1962). Transportation of infected seed tubers can introduce new genotypes of P. infestans into other potato-producing areas (Lambert et al. 1998). Tubers become infected when sporangia and zoospores produced on leaf and stem lesions are washed into the soil (Lacey 1967a, 1967b). Fungicide applications to control foliar late blight and vine killing using desiccants prior to harvest are the most common methods of reducing tuber infection (Stevenson 1993). However, research reports have indicated high levels of tuber infection even with numerous applications of protectant and curative fungicides (Flier et al. 1998; Fry and Goodwin 1997). Thus, tuber blight has become a major concern for seed and ware potato production, because of heavy losses in storage. The increased severity of tuber blight has been attributed to new strains of P. infestans that are more aggressive to tubers both in the field and in storage, and are difficult to control (Fry and Goodwin 1997; Lambert and Curie 1997). Managing foliar late blight is crucial in reducing tuber blight and rot. However, the relationship between levels of foliar blight and incidence of tuber blight has not been adequately documented. A high correlation between severity of blight in the foliage and tubers has sometimes been reported (Lapwood 1977), whereas other studies have observed either high incidences of tuber blight at harvest at low levels or in the absence of foliage blight symptoms (Hirst and Stedman 1962) or low incidence of tuber blight after high levels of foliage epidemics (Hirst et al. 1965; Olanya et al. 2002). A considerable amount of research has been conducted to determine factors affecting epidemics of foliar late blight (Harrison 1992). However, comparatively few studies have

been conducted on modeling factors responsible for tuber infection and development (Bain and Möller 1999). The effects of environmental conditions that impact sporulation on the foliage, inoculum dissemination, and subsequent survival of P. infestans in the soil in relation to tuber infection have not been thoroughly investigated. In addition, the effects of other soil variables, such as soil moisture and soil temperature, on tuber infection are also poorly understood. The importance of moisture for sporulation, zoosporogenesis, sporangia and zoospore germination, and survival has been documented (Hirst and Stedman 1962). It has also been documented that temperature influences spore germination (Crosier 1934), inoculum production (Sato 1994), and survival (Sato 1979). Because foliage blight often contributes to tuber blight, an understanding of the relationship between foliage blight and tuber infection is important for the management of tuber blight. Therefore, the objectives of this study were (i) to determine the effects of foliage blight levels on the incidence of tuber blight and (ii) to evaluate the relative importance of atmospheric factors, soil variables, and cultivar resistance in determining the risk of tuber blight infection.

Materials and methods Field site, experimental design, and disease establishment in New York Experiments were conducted during 1998 and 1999 at Freeville, New York, at the Homer Thomson Research Farm of Cornell University. Soils at the research farm are gravely loam soil (loamy, skeletal, mixed mesic, Glossoboric Hapludalf). Certified potato seed pieces, weighing 50–70 g each of ‘Allegany’, ‘Katahdin’, and ‘NY101’, were machine planted in plots 4 m × 3.7 m at spacing of 30 cm within the rows and 1 m spacing between the rows. Planting was done on 3 June 1998 and on 24 June 1999. The genotypes were chosen based on varying degrees of field resistance to both foliage and tuber blight (Dorrance and Inglis 1998; Inglis et al. 1996). Plots were laid out in a split-plot design with four replicates, with fungicide treatment and cultivar as the main plot and subplot, respectively. The main plots were 12 rows wide, and the subplots were four rows. To vary the levels of foliage blight, two fungicide application rates were applied: one-quarter concentration of the recommended dose and the recommended concentration (1.7 L/ha) of chlorothalonil

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Can. J. Plant Pathol. Vol. 29, 2007 Table 1. Description of variables used in analysis of the effect of atmospheric and soil factors on the incidence of tuber blight infection caused by Phytophthora infestans under field conditions in New York and Kenya. Category and variable description Precipitation Total precipitation (rainfall or irrigation)a Total days of precipitation in a growing season Soil temperature Mean soil temperaturea Total days with soil temperature 20 °C Foliage blight Area under the disease progress curve (AUDPC) Precipitation × soil temperature Precipitation with soil temperature 20 °C Precipitation × foliar blight Total precipitation during epidemics (initial and last day of disease assessment) Days of precipitation during epidemics Tuber resistance Tuber resistance index based on tuber blight F

df

F

P >F

3 2 2 1

3.17 2.54 37.8 45.8

0.0313 0.0808 0.001** 0.001**

3 3 3

2.01 70.9 49.1

0.1155 0.0001** 0.0001**

2 1 9 3

81.5 2.5 0.75 1.71

0.0001** 0.1156 0.6619 0.1665

3 3 3

2.02 1.09 17.4

0.1124 0.3545 0.0001**

1

2.62

0.1068

1 9

5.02 0.54

0.0261* 0.8469

3 3

18.5 4.29

0.0001** 0.0057**

4

0.36

0.9028

2

0.14

0.6547

3 2 2 1

2.98 1.61 25.9 7.66

1

0.15

0.7035

4 2

2.13 1.23

0.077 0.3006

2

0.24

0.0321 0.1721 0.0001** 0.006**

0.7865

Note: Units for area under the disease progress curve (AUDPC) are percent disease days. Tuber blight was assessed from tubers at a soil depth of 7 cm from the top of the ridge. Asterisks indicate significant P values: *, P < 0.05; ** P < 0.01. a Foliar application of protectant fungicide Bravo (a.i. chlorothalonil) consisted of application at 25% and 100% of recommended rate of 1.7 L/ha in New York. In Kenya, Dithane M45 (a.i. mancozeb) was applied at intervals of 7, 14, and 21 days. Fungicide treatments were compared with untreated controls in both countries. b In New York, ‘Allegany’ and ‘NY101’ (an advanced line) are moderately resistant to foliar blight, and ‘Katahdin’ is susceptible. In Kenya, ‘Tigoni’ and ‘Asante’ are moderately resistant to foliage blight, and ‘Dutch Robijn’ and ‘Kerr’s Pink’ are susceptible. c The cropping seasons were 2000 short rain, 2000 long rain, and 2001 long rain. d Kabete and Tigoni sites in Kenya.

relative humidity) for the presence or absence of lesions typical of tuber blight. In Kenya, because of low tuber blight incidence, tuber evaluation was only done at harvest. Tubers with external symptomatic lesions were sliced to confirm the presence of brown discoloration, typical of late blight tuber infection. Tuber blight was calculated as the percentage of total tubers per plot showing late blight symptoms. Soil and atmospheric microclimatic data in New York and Kenya Soil temperature was recorded using temperature sensor connected to data loggers (Watchdog dataloggers, Model 250, Spectrum Technologies Inc., Plainfield, Ill.) placed at depths of 15 cm below ground relative to top of the ridge in New York. Daily rainfall, air temperature, and relative humidity were obtained from the meteorological station at the Homer Thompson Research Station, 1 km from the experi-

mental site. The location of the site was representative and had similar topography to the field plots. In Kenya, soil temperature and moisture were recorded using a soil temperature and Water Mark soil moisture sensors, respectively (WatchDog dataloggers, Model 450; Spectrum Technologies Inc.) at a depth of 15 cm relative to the top of the ridge. Other weather variables, including daily rainfall, air temperature, and relative humidity, were obtained from temperature – relative humidity internal sensors and a tipping bucket sensor placed at the experimental sites (WatchDog dataloggers, Model 450; Spectrum Technologies Inc.). Statistical analysis Favorable and unfavorable soil temperatures and precipitation (>5 mm) events and their interactions used in analysis were defined (Table 1) based on previous research (Bain and Möller 1999; Crosier 1934; Fairclough et al. 1993; Hirst et al. 1965; Lacey 1966, 1967). In addition to foliage


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Table 3. Disease severity (area under disease progress curve; AUDPC) of potato cultivars inoculated with Phytophthora infestans (US-8, A2) at the Homer Thomson Research Farm, Freeville, New York, and protected with different application rates of Bravo (chlorothalonil) in 1998 and 1999. AUDPC Treatment

Cultivar

1998

1999

Control (untreated)

‘Allegany’ ‘Katahdin’ ‘NY101’ Mean LSD (α = 0.05)

624 892 737 751 48

742 956 815 838 87

(30)


580 813 678 690 66

714 933 764 804 54

(57)


535 789 651 658 43

698 890 760 783 74

(41)

25%

100%

Note: Foliar application of protectant fungicide Bravo (a.i. chlorothalonil) consisted of untreated control, and application at 25% and 100% of recommended rate of 1.7 L/ha. AUDPC values are calculated based on visual disease assessments (%) over time. ‘Allegany’ and ‘NY101’ (an advanced line) are moderately resistant to foliar blight, and ‘Katahdin’ is susceptible. LSD (least significant difference) values are for comparison of means within years. LSD values in parenthesis are for comparing differences in mean AUDPC values in 1998 and 1999.

blight levels (area under the disease progress curve; AUDPC), tuber resistance index of the cultivars based on previous publication (Dorrance and Inglis 1998), atmospheric variables, and soil variables were used in determining their relationships to tuber blight incidence. Linear, stepwise multiple regression, and correlation analyses were used to model the effect of foliar blight and other variables on incidence of tuber blight. Data from disease severity assessments were used to compute the AUDPC as described by Shaner and Finney (1977). The analyses of AUDPC, atmospheric variables, and soil variables were conducted to identify variables best explaining tuber blight incidence of each cultivar or the means across cultivars at different fungicide application levels. Direct and indirect effects of selected variables were determined using path coefficient analysis (Kendall and O’Muircheartaigh 1977; Li 1975). The final models were selected based on adjusted R2 and Mallow’s C statistic, in which models with C values close to the number of variables were chosen to avoid bias and multicollinearity (Netter et al. 1996). The effects of fungicide treatments and cultivar resistance on foliar disease severity (AUDPC) and tuber blight incidence were analyzed as a split-plot design using the general linear models (GLM) procedure analysis of variance (ANOVA) in SAS with fungicide treatment and cultivar as the main plot and subplot, respectively. Prior to analysis, percentages of tuber blight incidence were transformed using the arcsine square root to normalize variances. Variation of foliage blight over time was analyzed in SAS using the GLM procedure with the repeated measures option. Treatment means were com-

pared using Fisher’s protected least significant difference (LSD) test at α = 0.05. All analyses were performed using SAS version 9.1 (SAS Institute Inc., Cary, N.C.).

Results Late blight severity in New York Symptoms of late blight were observed in both 1998 and 1999 at 5 days after inoculation, and there was a rapid late blight progress on ‘Katahdin’ compared with the ‘Allegany’ and ‘NY101’ in all fungicide treatments (Fig. 1) in 1998 with significant (P < 0.0001) differences in disease over time for all the cultivars. A similar trend in disease progress on the three genotypes was observed in 1999 (data not shown). Fungicide treatments did not significantly (P = 0.0808) affect disease severity in both years of the study (Table 2). However, there was significant (P < 0.0001) difference in disease severity among cultivars: ‘Allegany’ showed lower foliage infection compared with ‘NY101’ and ‘Katahdin’ (Table 3). In addition, there was no significant (P = 0.9028) cultivar × fungicide treatment interaction on disease severity (Table 2). Across cultivars, disease severity was significantly higher in 1999 compared with 1998 in all the fungicide treatment plots (Table 3). Late blight severity in Kenya Fungicide treatments had highly significant (P = 0.0001) effects on disease severity (Table 2) with the lowest disease severity being recorded in the plots with 7 day application intervals. The highest levels of disease severity were ob-

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Can. J. Plant Pathol. Vol. 29, 2007 Table 4. Foliar blight blight severity (areas under disease progress curve; AUDPC) of four potato cultivars naturally infected with Phytophthora infestans (US-1, A1) in the field at Tigoni and Kabete in Kenya and protected with Dithane M45 (mancozeb) at different application intervals for potatoes grown during the 2000 long rainy (LR; March–May) and short rainy (SR; November) and the 2001 LR growing seasons. Interval (days) Untreated

7

14

21

Tigoni AUDPCb

Kabete AUDPCb

Cultivara

2000 LR

2000 SR

2001 LR

2000 LR

2000 SR

2001 LR

‘Asante’ ‘Tigoni’ ‘Dutch Robijn’ ‘Kerr’s Pink’ Mean LSD (α = 0.05)

314 227 324 486 338 102

681 644 855 1022 800 285

757 714 810 1182 866 163

(113)

84 22 105 327 134 133

412 463 579 1605 765 140

1207 811 1465 1589 1268 232

(120)


13 15 57 26 28 23

38 25 75 177 79 51

213 186 219 234 213 115

(106)

18 5 16 81 30 33

45 39 82 626 198 157

76 34 127 204 110 108

(90)


177 124 140 115 139 130

194 105 161 868 332 122

408 146 278 874 427 369

(174)

27 11 125 179 85 134

73 130 265 1624 523 272

359 151 536 727 443 137

(143)


118 61 117 179 119 50

460 136 114 940 413 221

737 339 769 744 647 170

(133)

73 69 80 293 128 108

285 292 494 1566 659 213

331 138 571 862 476 265

(154)

a

‘Tigoni’ and ‘Asante’ are moderately resistant to foliage blight, and ‘Dutch Robijn’ and ‘Kerr’s Pink’ are susceptible. AUDPC values are calculated from visual assessment of foliar blight severity (%) over time. LSD (least significant difference) values in parenthesis are for comparing differences in AUDPC between seasons at each experimental site. b

served in the untreated plots and in plots with 21 day application intervals (Table 4). Highly significant (P < 0.0001) cultivar effects on late blight severity were observed at Tigoni and Kabete during the growing seasons in 2000 and 2001 (Table 2). In both growing seasons, higher levels of disease severity were observed on ‘Kerr’s Pink’ compared with the other cultivars, and the lowest disease severity levels were observed on ‘Tigoni’ (Table 4). There was no significant (P = 0.6619) cultivar × fungicide interaction with ‘Kerr’s Pink’ having the highest levels of disease severity and more rapid disease development across all fungicide treatments at Kabete (Fig. 2). A similar trend in disease severity of cultivars and disease development across fungicide treatments was observed at Tigoni (data not shown). There were significant (P = 0.0001) differences in disease severity among seasons with more disease being recorded during the LR season in 2001 (Table 4). In both years, rapid disease progress was observed in the untreated plots at both Tigoni and Kabete (Fig. 3) compared with all the other fungicide treatments. Except during the 2001 LR season at Kabete, distinct differences in fungicide treatments were more apparent towards the end of the season (Fig. 3).

Tuber blight development at New York and Kenya sites In the New York experiments, fungicide applications did not significantly (P = 0.1721) affect the incidence of tuber blight at harvest (Table 2). Similarly, there were no significant differences in tuber blight incidence between the 2 years of the study (Table 2). However, both cultivars and depth of tuber setting had significant (P < 0.001) effects on the incidence of tuber blight (Table 2). Irrespective of the depth of tuber setting, ‘Katahdin’ had a significantly (P < 0.05) lower incidence of tuber blight compared with ‘Allegany’ and ‘NY101’. For example, in the untreated plots, the incidence of tuber blight at a tuber setting depth 0.05) correlated to foliage late blight levels in the foliage in New York. Soil temperatures 0.45) and significant association with tuber blight incidence were the number of days when soil temperature was 20 °C, and precipitation when soil temperatures are >20 °C. Generally, no consistent correlation was observed between soil moisture and the incidence of tuber blight. In Kenya, soil moisture, foliage disease levels, total precipitation, and frequency of precipitation during epidemics were the only variables that had significant correlations (0.248 < r2 < 0.466) with tuber blight incidence. Path coefficient analysis (Table 7) revealed that the number of days when soil temperature was 16–18 °C had a more significant (P < 0.01) direct effect on tuber blight incidence than other soil or atmospheric variables at the experimental site in New York (0.611) and Kenya (3.174). Days of precipitation during epidemics also showed similar significant direct effects on the incidence of tuber blight at New York (0.365) and in Kenya (0.711). Among the 14 variables evaluated, stepwise multiple regressions identified

AUDPC, total days of precipitation when temperature was 16–18 °C, and frequency of precipitation during epidemics as variables that individually or in association accounted for variation in tuber blight incidence (Table 7). Relationship between incidence of tuber blight with soil and atmospheric variables Cultivar resistance, tuber depth, frequency of precipitation during epidemics, and number of days when soil temperature was 16–18 °C were the most significant variables for predicting tuber blight incidence under epidemic conditions in New York (Table 8). In contrast, in Kenya, mean soil temperature, tuber resistance, and precipitation were the significant factors in determining the incidence of tuber blight (Table 8). The predictive regression models for tuber blight at different fungicide application levels had greater R2 values in New York (0.44 < R2 < 0.61) than in Kenya (0.40 < R2 < 0.46). In Kenya, parameter estimates for the regression model were not significant (P > 0.05) when weather variables were used to predict incidence of tuber blight (Table 8). Cultivarspecific regression models resulted in more consistent and significant prediction of tuber blight across fungicide treatments in New York, but a less consistent prediction was ob-

382 Fig. 4. Rainfall, soil temperatures, and atmospheric temperatures recorded during potato late blight epidemics observed in Freeville, New York, in the 1998 and 1999 growing seasons. Although the dates along the x axis for 1998 and 1999 do not correspond exactly, they allow easy comparison between the 2 years.

served for specific cultivars in Kenya (Table 9). At the New York location, the R2 values, intercept, and slope were similar among cultivars and among fungicide rates (Table 9). At the Kenyan sites, parameter estimates for the regression models were also similar among cultivars of varying resistance and fungicide application rates.

Discussion We determined the effect of foliar blight on the incidence of tuber blight and evaluated the effectiveness of foliar blight resistance, soil variables, and atmospheric variables in development of tuber blight under temperate conditions in New York and tropical highland conditions in Kenya. This study provides specific quantitative information on the effects of different pathogen genotypes, host resistance, and soil and atmospheric variables on tuber infection by P. infestans and provides avenues for developing models to predict occurrence of potato tuber blight under a wide range of environmental conditions. The relationship between foliage blight levels and tuber blight incidence was variable, and the resistance of potato cultivars to foliar blight did not affect incidences of tuber blight in a consistent manner. For example, at

Can. J. Plant Pathol. Vol. 29, 2007 Fig. 5. Rainfall, soil temperatures, and atmospheric temperatures recorded during potato late blight epidemics observed at Kabete, Kenya, during the 2000 short rainy (SR) and 2001 long rainy (LR) growing seasons.

the New York location, ‘Katahdin’, a cultivar with moderate susceptibility to foliar blight had lower tuber blight incidence compared with ‘Alleghany’ or ‘NY101’. Similarly, in Kenya, cultivars with resistance to foliar blight, such as ‘Tigoni’ and ‘Asante’, did not always have the lowest tuber blight incidence. This indicates that foliar resistance alone may not be a good predictor of the incidence of tuber blight. Previous findings by Platt and Tai (1998) revealed that resistance to late blight of foliage was not always linked to tuber blight. A similar conclusion was also drawn by Dorrance and Inglis (1998). In other studies, susceptibility of potato to foliar and tuber blight was observed to be affected by cultivar characteristics (Kirk et al. 2001). Under New York conditions, there was a negative correlation between tuber blight and foliage blight levels; however, in Kenya, there was a positive correlation. Even though inoculum levels were not measured in this study, these results may indicate that inoculum supply and duration are important factors in tuber infection. Under Kenya conditions, longer epidemics showed a positive correlation, whereas the fast epidemics in New York had a negative correlation, because they reduced the duration for inoculum production. There was considerable variation in the specific soil and climatic variables and the combination of these set of variables in explaining tuber blight incidence. Incidence of tu-


383

Table 7. Path coefficients between incidence of tuber blight and environmental variables. New Yorka

Path of association AUDPC versus tuber blight Direct effects Indirect effects Via days of precipitation when foliage LB 5%–50% Via total precipitation when foliage LB 5%–50% Via number of days at soil temperatures of 16–18 °C Via total precipitation at soil temperatures of 16–18 °C Overall correlation

–0.229 ns –0.216 0.214 0.149 0.001 –0.081

Total days of precipitation during epidemics versus tuber blight Direct effects 0.324* Indirect effects Via total precipitation during epidemics 0.235 Via number of days at soil temperatures of 16–18 °C 0.041 Via total precipitation at soil temperautres of 16–18 °C 0.035 Overall correlation 0.636 Total precipitation during epidemics versus tuber blight Direct effects Indirect effects Via days at soil temperatures of 16–18 °C Via total precipitation at soil temperautres of 16–18 °C Overall correlation

0.365** –0.04 –0.069 0.256

Total precipitation with soil temperature 16–18 °C versus tuber blight Direct effects 0.308 ns Indirect effects Via days precipitation during epidemics 0.067 Via total precipitation during epidemics 0.069 Overall correlation 0.445 Days with soil temperature 16–18 °C versus tuber blight Direct effects

0.611**

Kenyaa 0.399* –0.124 0.067 1.194 –1.204 0.332 0.072 ns –0.299 –1.018 3.283 –2.038 0.711* 0.228 –1.706 –0.767 –3.386** 0.036 0.689 –2.661 3.174**

Note: Path analysis is based on tuber blight incidence for experiments in 1998 and 1999 at New York and tuber blight incidence for three seasons and two sites for experiments during 2000 and 2001 in Kenya. Precipitation is the sum of normal rainfall and applied irrigation water. AUDPC, area under disease progress curve; LB, late blight severity (%). a Path coefficients are standardized regression coefficients between variables and tuber blight incidence, and asterisks show significant correlations: *, P < 0.05; **, P < 0.01; ns, not significant (P > 0.05).

ber blight was positively and significantly correlated with lower soil temperatures and days of precipitation or rainfall events. Significant correlation of tuber blight with soil temperature was detected in New York but not in Kenya, which may be the result of fewer number of days when soil temperatures during the cropping season were 0.05).

ducted by Sato (1979) who reported that soil temperature during and immediately after rain had greater effect on frequency of tuber rot than amount of rain. Stepwise multiple regressions explained more of the variation in tuber blight incidence under New York conditions with temperate climate but was not as effective with the data from tropical conditions in Kenya. This may be because several other factors, such as disease levels or environmental and soil microbial flora, that may vary among locations, can influence tuber blight infection and development. The dynamics of late blight progress and cultural practices may also have been a contributing factor to variation in tuber blight infection. In this study, the significantly lower incidence of tuber blight with increasing soil depth is similar to results reported by Porter et al. (2005) who observed higher levels of tuber infection close to the soil surface and very low levels of infection at 5 cm or deeper in the soil. This may be the result of a reduction in the concentrations of sporangia and zoospores at greater soil depths (Dubey and Stevenson 1996). The greater tuber infection detected on tubers harvested at a shallower soil depth suggest that soil depth of tuber or hilling practices may reduce tuber blight infection. Another possibility is that top tubers trap spores as they are washed down through the soil thereby limiting the infection of lower-set tubers. The high incidence of tuber blight observed at Freeville, New York, compared with Kenya may be attributed to the conditions favorable for the infection process or differences in pathogen virulence. During the cropping cycle, soil tem-

peratures in field plots in New York were