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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 9, Num. 1, 2001, pp. 209-213
African Crop Science Journal

African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 209-213

Reastions of Some Potato Genotypes to Late Blight in Cameroon

D. K. Njulaem, P. Demo, H. A. Mendoza, J. T. Koi and S. F. Nana
Cameroon Potato Programme, P. O. Box 80, Bamenda, Cameroon

Code Number: CS01050

ABSTRACT

Field experiments were conducted in Cameroon in 1995 and 1996 to evaluate reactions of different potato genotypes to late blight. There were significant differences among genotypes for tuber yield, late blight readings and earliness. The application of fungicides significantly increased potato yield but had a non significant effect on the damage due to foliage blight (Phytophthora infestans). The limited effect of fungicide treatment was because the late blight severity was so high that the eight fungicide applications could not provide sufficient level of disease control and/or the persistence rain throughout the growing season might have washed-off the applied fungicide, thus decreasing its effect. There was a significant year effect on the yield. The fifth late blight reading (LB5) was more related to yield than the later readings suggesting that the damage caused by the disease at an earlier stage of plant development contributed most to yield reductions. Several advanced genotypes had similar or superior performance to that of CIPIRA and TUBIRA released in Cameroon in 1992.

Key Words: Cameroon, Phytophthora infestans, potato yield, Solanum tuberosum

RÉSUMÉ

Des essais en champs ont été conduits au Cameroun en 1995 et 1996 pour évaluer des réactions de différents génotypes de pomme de terre au mildiou. Il y avaient des différences variétales significatives pour le rendement des tubercules, les cotes du mildiou et la précocité. L'application des fongicides a augmenté significativement le rendement des tubercules mais sans effet sur les dégats causés par le mildiou foliaire. L'effet limité du traitement au fongicide était dû au fait que la sévérité était très élevée de telle sorte que les applications des fongicides ne pouvaient pas produire un niveau suffisant du contrôle de la maladie, et/ou la persistance des pluies pendant toute la saison pourrait avoir effacer les fongicides pulverisés, ce qui réduisit leurs effets. L'effet de l'année était significatif ainsi la septième lecture du mildiou et la précocité. La cinquième lecture du mildiou (LB5) était plus associée au rendement plus que LB4,LB6 et LB7, suggérant que la pratique d' utiliser les dernières lectures des dégats du mildiou pour l' associer à la réduction du rendement pourraient ne pas être appropriée comme les dégats de la maladie au premier stade de développement de la plante peuvent être significatifs pour l'interprétation des résultats. Plusieurs génotypes avancés avaient des performances similaires ou supérieures à celles de CIPIRA et TUBIRA diffusées au Cameroun en 1992.

Mots Clés: Cameroun, Phytophthora infestans, rendement du pomme de terre, Solanum tuberosum

INTRODUCTION

Potatoes (Solanum tuberosum L.) in Cameroon are mainly cultivated in the highlands of the North West, South-West and West Provinces. It is estimated that potatoes occupy 70,000 ha and that 95 percent of the production area is located between 1000 to 2000 meters above sea level. In mixed cropping systems that predominate in the production areas, average yield is low, 1.5 to 20 t ha-1. Productivity is higher in the predominantly monocropped market gardens of the West and South-West provinces averaging 7 to 12 t ha-1.

The low productivity is attributed to several factors, the most important being attack by Phtophthora infestans, causal agent of late blight. Most of the cultivated varieties such as CAR-DINAL, DIAMANT, FAMOSA DESIREE, KONDOR and ROMANO are very susceptible to the disease, and the situation is made worse because the environmental conditions are very favourable to the disease. Additionally, the average farmer has limited capacity to protect their crops through use of fungicides.
This study evaluated the response of 23 potato clones to late blight in two consecutive years in a representative area of potato production in Cameroon.

MATERIALS AND METHODS

Twenty three advanced clones and four commercial varieties (checks) were tested during 1995 and 1996 in the Upper Farm Experimental Station in Cameroon. Two of the checks, DESIREE and FAMOSA were of European origin, while the other two CIPIRA and TUBIRA were local varieties.

Most of the clones including the checks CIPIRA and TUBIRA originated from crosses between late blight resistant parents of Mexican origin crossed either to another Mexican blight susce-ptible or moderately resistant progeny. They were originally selected at the International Potato Centre (CIP) in Lima, Peru, mainly for immunity to potato viruses X and Y and/or heat tolerance, earliness and bacterial wilt resistance. These genotypes are thought to possess R-genes for resistance to late blight.
Upper Farm is located at 2000 meters above sea level in the highlands of the North-West Province. Annual precipitation averages 2200 mm mainly concentrated between April and September. Temperature fluctuates between 10.3 and 21.1°C. The soils are relatively low in nitrogen and poor in phosphorus, requiring a fertiliser dose of 120 N: 180 P: and 100 K per hectare.
Two experiments were carried out within each year, one protected with 8 applications of a maneb-type fungicide (Dithane M45) against late blight and the other unprotected. All experiments were conducted using a randomised complete block design. The experiments with fungicides had 4 replications with 20 plants per genotype per replicate. The unprotected trial had 6-replications with 10 plants per experimental unit in 1995 and 15 in 1996.

Seven weekly spaced readings of late blight severity were made using the CIP's 9 point scale based on percent foliage damage as follows; 1 =no lesions, 2 = 2.5%, 3 = 10%, 4 = 25%, 5 = 50%, 6 = 75%, 7 = 90%, 8 = 97.5%, and 9 = dead plant (Henfling, 1987). Earliness was also rated on a 9 point scale based on the stage of foliage development with 1= very late in maturity (plant completely green, starting flowering) and 9 = early maturity (plants showing symptoms of senescence).
For purpose of statistical analysis it was assumed that the effects of years and genotypes were random and that of treatments (fungicide and no-fungicide) was fixed. Statistical analysis of experimental data was made using SAS. Each experiment was analysed using the ANOVA procedure, but the combined analysis was made using the GLM procedure basing on stepwise procedure, to determine which of the variables included in the study were more related to yield.

RESULTS AND DISCUSSION

Table 1 presents the overall statistical analysis showing the main effects of years, treatments and genotypes, the first and second order interactions and the experimental error. The analysis shows several important features. First, the mean squares for the year effect were very large for all characters and in all cases were significant at the 1% probability level. Second, the treatment (fungicide application versus no-fungicide application) significantly influenced yield but was insignificant in other respects, particularly in foliage blight damage. Third, there were significant differences among genotypes for all the traits considered in the study. And, fourth, while the first order interactions Yx T, T x G and the second order interaction Y x T x G were in most cases not significant, the interaction Y x G was highly significant for most of the traits studied indicating a differential behavior of genotypes in the two years.

Table 2 shows the strong year effect. An average yield of 34.9 t ha-1 in 1995 dropped to 23.3 t ha-1 in 1996. The average late blight score at the 7th reading growth stage increased from 3.6 (near to 25% of foliage damage) in 1995 to 6.9 (about 90% of foliage damage) in 1996. The overriding year effect on all the characters studied were closely linked to the high late blight severity in 1996. In 1996, inspite of a total of 8 sprays of Dithane M45, late blight severity was still high in the sprayed plots, with significantly more attack in the unsprayed plots. The year effect was also significant for average earliness that went from 4.9 in 1995 to 5.9 in 1996. This meant that the plants "matured" earlier in 1996 as late blight shortened the length of foliage life. The effect of genotypes (Table 2) and year by genotype interaction were also significant (Table 1). The significant interaction led to differential ranking of clones in the two years. For instance, the clone 386294.4 that was an average yielder in in 1995 was the top yielder in 1996, with a yield of nearly 35 t ha-1.

Table 3 shows the average weekly late blight readings of fifth, sixth and seventh recording in 1995 and 1996 as well as yield of each advanced clone and variety. The blight disease evolution in each clone with exception of 386005.3, 388724.3 and 387096.11 was higher in 1996 than in 1995 such that the early maturing genotypes 387633.2, LM-89-8.3, FAMOSA and 377957.5 had 90% or more of their foliage destroyed at the sixth recording (LB6). However, these clones yielded equally or more than several clones that had significantly less blight damage. It is likely that an early tuber initiation followed by an early bulking permitted these clones to accumulate dry matter before their foliage were seriously damaged by blight. A similar situation has been described in Uganda by Kankwatsa (2001). In the Uganda situation, higher yields were obtained inspite of higher levels of late blight attack in the early planted potatoes because the disease reached severe proportions after bulking. The late planted potato had lower late blight attack but yielded less than the more severely blighted early season crop. Contrastingly, other genotypes of equivalent earliness such as DESIREE, 386297.1 and LM-89-37.2 had lower yield because they were more susceptible to the disease at an earlier stage of development while the former group had a mode-rate resistance at the early stages of development.

The correlation coefficients between the 1996 yield and fifth, sixth and seventh late blight readings were 0.563**, -0.491** and -0.478**. The correlations between earliness and fifth, sixth and seventh late blight readings were very high, 0.874***, 0.928*** and 0.955***, respectively. The correlation between yield and earliness (-0.401) was significant at 0.05 probability level. Therefore, to interpret the significance of late blight foliage damage on yield, earliness must also be considered.
Additionally, the fifth late blight reading (LB5, early bulking) was more closely related to yield reductions than, bulking period LB6 and LB7, suggesting that the common practice of using the last (late season) readings of late blight damage and linking it to yield reduction was not appropriate since the damage occurs at an earlier stage of plant development. However, recordings at the earlier stage (LB4) also had weak correlation with yield, suggesting that LB5 was the most appropriate stage for assessment of late blight damage.

A number of conclusion can be drawn from this study. First, there were some genotypes that showed a high resistance under severe late blight conditions and yielded significantly more than the commercial varieties used as checks. Notable amongst these were clones 386005.3, 387093.5, 386297.4. Second, when environmental conditions are favourable for late blight attack, eight or more sprays of fungicides are necessary to control the disease. However, if there are facilities for environmental monitoring to allow for forecasting the likehood of late blight epidemics, less sprays are required (Krause et al., 1975; Namanda et al., 2000).Third, the use of early maturing varieties with moderate resistance to blight when planted at the right moment permit acceptable yields. This, however, is a challenge in tropical environments since opportunities for environmental monitoring and weather forecasting are limited.

REFERENCES

Kankwatsa, P. 2001. Integration of host resistance, cultural methods and fungicidal application for the management of potato late blight in Uganda. M.Sc. Thesis, Makerere University, Kampala. 140 pp.

Krause, R., Massie, L.B. and Hyre, R.A. 1975. Blitecase: a computerised forecast of potato late blight. Plant Disease Reporter 59:95-98.

Henfling, J.W. 1987. Late blight of potato. Technical Information Bulletin 4. CIP, Lima, Peru. 22 pp.

Namanda, S., Adipala, E., Olanya, M., El-Bedewy, R., Hakiza, J.J. and Bhagsari, A.S. 2000. Integration of host resistance and minimum fungicide application for management of potato late blight in southwestern Uganda. In: Fourth Regional Meeting of the Forum on Agricultural Resource Husbandry, 10 - 14 July, 2000. pp. 163 - 165.

TABLE 1. Mean squares of the overall analysis of variance for various potato traits,1995 and 1996

Source

df

Earliness

LB7

Yield

Rep/year

10

1.2ns

1.5

36.1ns

Year (Y)

1

128.9**

1287.1**

14069.1**

Treatment (T)

1

71.7ns

3.2ns

406.8*

Genotype (G)

26

71.3**

29.9*

421.0**

Y x T

1

8.3ns

7.0**

46.7ns

Y x G

26

15.2**

4.3**

120.5*

T x G

26

4.0ns

0.9ns

60.2ns

Y x T x G

26

2.4*

0.9ns

46.0ns

Error

422

1.5

0.7

76.9

CV(%)

14.2

16.3

7.4

LB7 = Late blight severity assessed approximately

TABLE 2. Reaction of some potato genotypes to late blight earliness and yield obtained in Cameroon in 1995 and 1996
Gentypes
Late blight1
Earliness2
Yield (t ha-1)
Yield loss (%) (1996)

1995

1996

1995

1996

1995

1996

377957.5

5.4

8.9

8.0

9.0

32.5

23.5

27.8

382119.20A

2.0

6.0

2.8

5.0

30.2

22.4

25.8

384321.16

2.4

6.0

3.8

4.8

37.3

26.1

30.2

386005.3

4.2

5.4

4.0

3.2

39.5

31.4

20.5

386297.1

5.3

9.0

5.8

9.0

30.4

18.4

39.4

386297.4

3.6

5.5

6.4

4.0

34.8

35.8

-2.8

387091.37

2.4

5.0

7.2

3.8

31.0

25.6

17.3

387093.5

2.4

6.6

4.8

5.8

46.1

30.0

34.9

387096.11

2.3

9.8

2.0

1.6

32.8

23.7

27.7

387172.4

3.2

7.0

4.8

6.0

41.0

25.8

37.0

387494.8

2.7

6.2

3.0

5.4

44.1

24.2

45.3

387633.2

2.8

8.3

4.6

8.6

41.0

23.4

42.8

387795.2

3.1

6.6

5.4

5.6

47.2

24.8

47.5

388724.3

2.8

5.9

1.8

2.4

39.2

22.9

41.6

389561.5

2.7

6.7

2.2

5.6

31.5

17.0

45.9

389573.4

2.7

5.6

3.4

3.0

34.9

19.8

43.3

389614.2

2.8

7.3

5.2

6.0

33.1

20.6

37.7

389650.9

2.2

6.2

1.6

5.0

27.2

20.0

26.7

720050

5.0

6.8

5.4

5.4

31.6

22.6

28.5

CIPIRA

2.0

5.6

3.2

4.6

33.9

26.6

21.6

DESIREE

6.8

9.0

8.6

9.0

32.8

14.2

56.6

FAMOSA

3.5

8.6

5.6

9.0

36.1

25.5

29.4

LM-89.37.2

5.3

8.4

8.2

8.0

19.9

16.8

15.2

LM-89-44.1

5.2

7.4

6.6

6.4

29.2

23.0

21.1

LM-89-48.2

4.6

8.6

6.2

8.0

30.9

19.0

35.7

LM-89.8.3

6.0

9.0

7.0

9.0

36.4

22.3

38.7

TUBIRA

3.1

6.0

3.4

5.2

36.4

24.0

34.0

Mean

3.6

6.9

4.9

5.9

34.9

23.3

CV (%)

13.9

4.7

17.6

10.6

6.9

8.0

LSD(0.05)

1.1

0.6

1.9

1.4

9.8

4.6

1Based on 1-9 CIP scale (Henfling, 1997)
2Based on a 9 point scale where 1 = very late maturity, 9 = early maturity

TABLE 3. Late blight (LB) severity and potato yield in Cameroon in 1995 and 1996

Genotype

LB5

LB6

LB7

Yield (t ha-1)

LB5

LB6

LB7

Yield (t ha-1)

387795.2

2.1

2.9

3.1

47.2

3.6

5.6

6.6

24.8

387093.5

1.8

2.2

2.4

46.1

3.8

5.5

6.6

30.0

387494.8

1.5

2.2

2.7

44.1

3.2

5.2

6.2

24.2

387172.4

1.5

2.4

3.2

41.0

3.7

5.7

7.0

25.8

387633.2

1.8

2.6

2.8

41.0

4.9

7.0

8.3

23.4

386005.3

1.9

2.8

4.2

39.5

3.2

5.0

5.9

31.3

388724.3

1.7

2.3

2.8

39.2

3.0

4.9

5.9

22.9

384321.16

2.0

2.2

2.4

37.3

3.6

5.1

6.0

26.1

TUBIRA

1.8

2.3

3.1

36.4

2.9

4.9

6.0

24.0

LM-89.8.3

3.9

5.0

6.0

36.4

5.6

8.2

9.0

22.3

FAMOSA

2.3

2.8

3.5

36.1

5.0

7.6

8.6

25.5

389573.4

1.8

2.1

2.7

34.9

3.1

4.5

5.6

19.8

386297.4

2.4

3.4

3.6

34.8

2.6

4.4

5.5

35.8

CIPIRA

1.5

1.8

2.0

33.9

2.8

4.6

5.6

26.6

389614.2

2.2

2.5

2.8

33.1

3.8

5.4

7.3

20.6

DESIREE

5.1

6.4

6.8

32.8

6.9

9.0

9.0

14.2

387096.11

1.5

1.7

2.3

32.8

2.9

4.1

4.8

23.7

377957.5

2.6

4.2

5.4

32.5

4.8

7.9

8.9

23.5

720050

2.1

3.7

5.0

31.6

4.2

5.7

6.8

22.6

389561.5

1.5

2.4

2.7

31.5

3.8

5.6

6.7

17.0

387091.37

1.7

2.2

2.4

31.0

2.3

4.1

5.0

25.6

LM-89.48.2

2.2

3.5

4.6

30.9

4.7

6.8

8.6

19.8

386297.1

2.7

3.7

5.3

30.4

5.3

8.3

9.0

18.4

38119.20A

1.7

1.9

2.0

30.2

3.3

5.0

6.0

22.4

LM-89.44.1

3.0

4.4

5.2

29.2

4.5

5.9

7.4

23.0

389650.9

1.5

2.1

2.2

27.2

3.6

5.2

6.2

20.0

LM-89.37.2

3.1

4.3

5.3

19.9

5.0

7.2

8.4

16.8

Mean

2.2

3.0

3.6

34.9

3.9

5.9

5.9

23.3

CV (%)

15.6

14.0

14.3

6.9

13.6

10.3

9.1

8.0

LSD (0.05)

0.6

0.7

0.9

9.8

0.5

0.5

0.6

4.6

LB5, LB6, LB7 correspond to late blight readings at initiation of bulking, mid-bulking, after bulking
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