+Bioline International Official Site (site up-dated regularly)

Potatoes are grown in Israel in two growing seasons, autumn and spring. Two foliar diseases, early blight (Alternaria solani) and late blight (Phytophthora infestans) threaten the crop and if not managed properly, may induce substantial yield losses. Concepts for the integration of genotype resistance, age-related resistance and fungicide type for the suppression of both pathogens were developed based on the epidemiology of the pathogens and in relation to their interactions with the host. The concepts were evaluated in 7 field experiments from 1994 to 1998. Analysis of disease progress curves revealed that in plots managed according to the integrated management concepts, early blight was suppressed as good as in plots treated with a specific fungicide against A. solani, and late blight was suppressed as good as in plots treated with specific fungicides against P. infestans. Less sprays and lower quantities of fungicidal active ingredients were applied in plots treated according to the integrated management strategy. Yield in the integrated management treatment did not differ, or was significantly higher, than yield in the other treatments.

Key Words: Alternaria solani, integrated disease management, Israel, Phytophthora infestans, Solanum tuberosum

Les pommes de terre sont cultivées en Israël pendant deux saisons culturales, l'automne, et le printemps. Deux maladies foliaires, la bactériose (Alternaria solani) et le mildiou (Phytophytora infestans) envahissent la culture et si elles ne sont pas bien gérées, elles peuvent causer des pertes énormes de rendement. Les concepts de l'intégration de la résistance génotypiques, résistance liée à l' âge et le type de fongicide pour la suppression des deux pathogènes ont été développés basés sur l' épidémiologie des pathogènes et en relation de leur intéractions avec l'hôte. Les concepts ont été évalués dans 7 champs depuis 1994 jusqu'en 1998. L'analyse de progression des maladies a montré que dans les parcelles gerées suivant les concepts de gestion, la bactériose a été suppressée comme dans les parcelles traitées avec un fongicide spécifique contre A. solani et le mildiou a été réduit comme dans les parcelles traitées evec un fongicide spécifique contre P.infestans. Les pulvérisations et les faibles quantités de fongicides à ingrediants actifs ont été appliquées dans les parcelles selon la stratégie de gestion intégrée n'ont pas été différentes ou étaient significativement élevées plus que les rendements dans les autres traitements.

Mots Clés: Alterinaria solani, getion intégrée des maladies, Israël, Phytophytora infetans, Solanum tuberosum

INTRODUCTION

Potatoes are grown in Israel (300 E, 310 N) in several regions. The main area of production (7,000 ha, 80% of the national cultivated area) is in the northern Negev. The climate there is semi-arid with mild winters and hot, rainless summers. There are two main seasons for potato production, autumn and spring. For the autumn season crop, potatoes are planted in late August - early September and harvested during December and January; for the spring crop they are planted in late January - February and harvested during late May, June and July. The growing seasons differ markedly in respect to the environment: for the autumn season crop, growth starts when temperatures are high and the days are relatively long, and continues under decreasing temperature, day length and radiation; for the spring season crop, growth starts when temperatures are relatively low and days are relatively short, and continues under increasing temperature, day length and radiation (Levi et al., 1986).

Early blight, caused by Alternaria solani and late blight, caused by Phytophthora infestans, are the main causes of defoliation of potato in Israel and elsewhere (Rotem, 1994). Severe epidemics of early blight may restrict potato yields by up to 20-30% and late blight may result in complete crop failure in severe epidemics (Rotem, 1994; Shtienberg et al., 1996). In order to suppress the diseases, fungicides are often applied to the foliage. During a typical growing season, potato fields are sprayed with fungicide eight to twelve times. Until recently only protectant fungicides (e.g., maneb, mancozeb and chlorothalonil) were available for the suppression of Alternaria. Several systemic fungicides of the triazole group (such as tebuconazole and difenoconazole) are now available for use. Application of these fungicides in potato resulted in a longer period of effectiveness and sometimes, led to superior disease suppression and to higher yields than those achieved by application of protectant fungicides (Shtienberg et al., 1996).

Results of previous studies (Christ and Mazuga, 1989; Fry and Shtienberg, 1990; Shtienberg et al., 1996) enable us to develop concepts for the integration of genotype and age-related resistance in the management of A. solani and P. infestans to reduce fungicide use. An attempt was made to develop a reliable and cost effective solution for changing environment and under uncertain conditions. The basic principles are as follows: late blight is the most important disease and the crop has to be protected before the pathogen has invaded the field. Thus, sprays are applied in a prophylactic manner as an insurance measure. The frequency of sprays is dependent on the response of the cultivar to P. infestans and on the season: sprays are applied less frequently to moderately resistant cultivars and in the autumn. To quantify the general risk a specific field is facing, three "risk" categories were defined. Risk category "A" implies that late blight was not observed at all in the region; risk category "B" that late blight was reported in the region and risk category "C" that symptoms were observed in the specific field. A regional system notifies growers on the current situation of the disease in the region. The type of fungicides to be applied and frequency of sprays is determined according to the current risk category. In general, protectant fungicides are to be applied unless the risk category is C and conditions are highly favourable to P. infestans (i.e., cool rainy weather). In that case, a more effective specific fungicide against late blight should be applied.

Early blight is to be considered specifically only in the autumn season employing the concepts that follow. Application of fungicides is not needed in plants at the vegetative stage because they are relatively resistant. Accordingly, spraying should be initiated only when host response to Alternaria shifts towards increased susceptibility, i.e., at the initiation of the reproductive stage (Shtienberg et al., 1989). The frequency of subsequent sprays should be determined according to the genotype resistance of the cultivar and the efficacy of the fungicide, in relation to changes in age-related resistance. Accordingly, protectant fungicides should be applied initially at relatively long intervals, which will shorten as the crop ages. Towards the end of the season, more effective control, by means of a systemic fungicide, is recommended. Genotype resistance will be considered by spraying moderately resistant cultivars less frequently than susceptible ones. In the spring, the environment promotes host growth and the production of new leaves precludes a significant reduction in yield. Towards the end of the season, if late blight alert B or C is issued, a reduced rate of the systemic fungicide used against early blight is mixed with a full rate of a fungicide against late blight (protectant or systemic). In the spring season, early blight is not considered specifically because the protectant sprays applied against late blight are sufficient for adequate suppression.

In this study we examined the concepts outlined above for the integration of genotype resistance, age-related resistance, and fungicide efficacy in the management of both early and late blight in potatoes.

METHODOLOGY

Seven experiments were conducted in the northern Negev region of Israel; 4 in the autumn season and 3 in the spring season. Trials were performed to evaluate the applicability and significance of the integration concepts. Disease suppression in plots treated according to the concepts of integration was compared with that achieved by routine applications of protectant or systemic fungicides against early or late blight. All experiments were conducted in commercial fields so that the results reflect the complexity and uncertainty prevailing in normal situations. In this report, results of 4 experiments are presented but those recorded in the other three experiments were similar. Two of the experiments were conducted in the autumn season of 1997/8 (exps. 1 and 2) and two in the spring seasons of 1996 (experiment 3) and 1997 (experiment 4). Certified potato seeds of the cultivars Mondial (exp. 1), Nicola (experiment 2), Alpha (experiment 3) and Ladi-Rosetta (experiment 4) were machine-planted in the experiments; these cultivars are susceptible to both pathogens. Plots consisted of four 7-m long rows. Inter-row spacing was 0.96 m and intra-row spacing of plants was 25-30 cm. Plots were separated from each other by fallow areas approximately 1 m wide. Fungicides (in 260-300 L water ha^-1) were applied by means of a motorised back-sprayer at a pressure of 275 kPa with cone-jet X6 nozzles. Sprays did not contain spreader, sticker, or adjuvant.

In each trial one protectant and one systemic fungicide (for each pathogen) were used. The protectant fungicide was mancozeb (Manzidan, 80% WP, Makhteshim Chemical Works, Israel) at a rate of 2.4 kg a.i. ha^-1. Systemic fungicides were: tebuconazole (Folicur, 25% EC, Bayer AG, Germany) at a rate of 0.187 kg a.i.ha^-1; cymoxanil+mancozeb (CM) (Mancur, 14.2+57.2%, WP, Sandoz, Switzerland) at a rate of 0.5+2.0 kg a.i.ha^-1 or Oxadixyl + cymoxanil + mancozeb (OCM) (Sandocur-M, 6.9+ 14.2 + 57.2%, WP, Sandoz, Switzerland) at a rate of 0.24+0.5+2.0 kg a.i. ha^-1.

The following five treatments were included in the trials: 1) untreated control; 2) management of both early and late blights by protectant fungicide (application of mancozeb on a 7-day schedule); 3) optimal management of early blight (application of tebuconazole on a 10-14-day schedule); 4) optimal management of late blight (application of CM or OCM on a 10-14-day schedule); and 5) application of protectant and systemic fungicides according to the concepts of the integrated management strategy. The number and the type of the fungicides applied in the various treatments are indicated in Table 1.

Disease was assessed visually independently by two individuals and the average scores were recorded. Assessments were made every 7-14 days from the appearance of disease symptoms until the end of the season. Since it was not possible in the field to distinguish between symptoms induced by A. solani or by P. infestans, their combined intensity was assessed. Thus, disease severity records reflect the intensity of both pathogens and towards the end of the season also the natural senescence of the crop. A. solani develop on older leaves and intensify the rate of their senescence. Severity records were used to calculate the Area Under the Disease Progress Curve (AUDPC) for each of the treatments. After crop maturity, a 5 m² area was harvested from the center of each experimental plot of all treatments. Tubers were collected by hand and weighed, and the yield per hectare was then calculated. Results were subjected to statistical analysis and where F values showed significant differences, Fisher's protected LSD test was applied at P = 0.05.

RESULTS AND DISCUSSION

Both early and late blights developed in all experiments, however, their intensity differed markedly among the experiments. Early blight was severe in both experiment 1 and 2 (the autumn experiments); late blight was severe in exp. 1 as well. In the spring (exp. 3 and 4), late blight rather than early blight was the predominant disease; early blight was minor and did not affect the foliage markedly until the end of the season.

Initially, early blight developed in experiment 1. However, as time passed, late blight intensified and plots treated with tebuconazole (which is not effective at all against P. infestans) were significantly more diseased than plots that were treated against both pathogens (in the integration treatment). Similarly, plots treated with CM were protected adequately against late blight, but severely infected by early blight (Table 2). Effect of the treatments on AUDPC and yield reflect the dynamic of the epidemics of both pathogens throughout the season. AUDPC value was significantly lower and yield was significantly higher in the integration treatment than in other fungicide treated plots. AUDPC values and yield recorded in the fungicide treated plots differed significantly from those recorded in the untreated control (Table 2).

Early blight was the predominant disease in experiment 2. Accordingly, plots treated with OCM alone were significantly more diseased (as reflected by the AUDPC values) than plots treated with tebuconazole or according to the integrated management strategy. Mancozeb, which control both pathogens decreased disease severity initially, but was significantly inferior to the tebuconazole treatment by the end of the season, when the host became highly susceptible to A. solani (Table 2).

In the two spring experiments (exps. 3 and 4) late blight was more destructive than early blight (although both disease prevailed in the experiments). Both pathogens were adequately suppressed in the OCM treatment and also in the integrated management treatment. This control was achieved in spite of the fact that only 1-3 OCM sprays were applied in the integration treatment as compare to 5 sprays in the OCM treatment. Disease (presumably late blight) control in the tebuconazole treatment of experiment 3 was inadequate, as reflected by the higher AUDPC values and lower yields (Table 3).

Development of more than one pathogen in a certain field is a well-known situation. Potato growers in the Negev region of Israel had to combat simultaneously A. solani and P. infestans. Management of the two pathogens is complicated for several reasons: the pathogens may differ in prevalence and potential damage; the response of the commercial cultivars to each pathogen may differ; the efficacy of fungicides against each pathogen may differ as well, some fungicides may be effective for the suppression of one pathogen but not for the other. Finally, because of the cost of the pesticides and due to environmental considerations, the use of chemical control should be limited as much as possibe.

This complex situation calls for development of integrated approach to crop management. All components have to be considered and formulated in a way that risks from uncontrolled epidemics are minimised. At the same time, application of unnecessary specific, expensive fungicides should be prevented. The assumption underlying the concepts for integration presented in this study is that the effects of different control measures are complementary and additive. Accordingly, application of one measure may compensate for a decrease in another measure. Integration of three measures was considered, viz., genotype resistance, age-related resistance and fungicide type and efficacy. Genotype resistance and age-related resistance were considered as measures in which their contribution is more or less predetermined. Fungicides were used as a flexible measure by which it was possible to respond to the disease situations in the field during the growing season.

The integrated management strategy presented in this study provided adequate suppression of both early and late blights in commercial fields and under diverse conditions. Calculation of the cost/benefit ratio revealed that the net income (after deduction of spraying expenses) was markedly higher in the integrated management treatment than in the other treatments (results not shown). Potato growers in the Negev region of Israel currently use components of the strategy with considerable success.

REFERENCES

TABLE 1. The number and type of fungicidal sprays applied in the experiments

	Experiment 1		Experiment 2		Experiment 3		Experiment 4
	Treatments¹
	T 2-4	T 5	T 2-4	T 5	T 2-4	T 5	T 2-4	T 5
No. of mancozeb sprays	9	7	7	3	8	8	10	5
No. of tebuconazole sprays	5	0	5	1	3	0	-	-
No. of OCM or CM sprays²	7	0	7	1	5	2	5	1
No. of tebuconazole+mancozeb sprays³	0	0	0	1	0	0	0	0
No. of tebuconazole + OCM/CM sprays	0	2	0	1	0	0	0	2

¹Number of sprays in treatments 2-5. Mancozeb was applied only in treatment 2; tebuconazole only in treatment
²CM = cymoxanil + mancozeb; OCM = oxydixly + cymoxanil + mancozeb
³ and OCM or CM only in treatment 4. In treatment 5 (the integration treatments) mancozeb, tebuconazole and OCM or CM (or their mixtures) were applied as needed

TABLE 2. Effects of the treatments on disease severity and on potato yield in the autumn experiments¹

Treatment	Experiment 1			Experiment 2
	Disease (%)	AUDPC	Yield (t ha^-1)	Disease (%)	AUDPC	Yield (t ha^-1)
Untreated control	100.0 a	2960 a	34.3 c	99.1 a	3197 a	37.2 b
Mancozeb	83.2 b	2113 b	41.8 b	81.2 ab	1996 b	43.2 a
CM/OCM	80.0 b	1884 bc	38.6 bc	91.7 a	2215 b	41.0 ab
Tebuconazole	74.5 b	1583 c	41.2 b	65.0 b	1608 c	42.2 ab
Integrated treatment	42.5 c	1081 d	50.2 a	57.5 b	1510 c	44.2 a

¹Disease severity was recorded 100 days after planting or integrated over the entire epidemic (AUDPC = Area under the disease progress curve). Numbers in each column followed by different letters differ significantly (at P < 0.05) as determined according to Fisher's protected LSD test

TABLE 3. Effects of the treatments on disease severity and potato yield in the spring experiments¹

Treatment	Experiment 3			Experiment 4
	Disease (%)	AUDPC	Yield (t ha^-1)	Disease (%)	AUDPC	Yield (t ha^-1)
Untreated control	50.0 a	1006 a	42.5 b	80.0 a	1074 a	35.7 b
Mancozeb	17.5 b	244 c	51.8 a	45.0 b	823 b	46.2 a
CM/OCM	20.0 b	327 c	51.8 a	32.5 b	683 b	46.6 a
Tebuconazole	45.0 a	588 b	45.5 b	-	-	-
Integrated treatment	20.0 b	315 c	51.8 a	37.5 b	696 b	50.0 a