Prevalence of apple scab and powdery mildew infecting apples in Uganda and effectiveness of available fungicides for their management

Apple scab caused by Venturia inaequalis (Cooke) wint, and apple powdery mildew, caused by Podosphaera leucotricha (Ell. and Ev.), are the most important diseases of apples in Uganda. Control of apple scab and powdery mildew require the application of fungicides in absence of resistant apple cultivars. This study was conducted to identify effective control fungicides and to document the status of these diseases in south western Uganda. The survey results of 2012 indicated that scab incidence was highest in districts of Kanungu (71.90%), followed by Kisoro (48.14%), Kabale (41.03%), Buhweju (29.23%), Mbarara (28.75%) and Rukungiri (17.80%). The severity of apple scab measured as percentage leaf area affected ranged from 20.12 to 76.19%. Powdery mildew incidence ranged from 30.00 to 70.00% with severity score ranging from 14.63 to 76.19%. Fungicides containing propineb (70% a.i), metalazyl (4% a.i) + macozeb (64% a.i), and bupirimate (25% a.i) were found to be effective at controlling apple scab with potential to reduce the disease severity by 61.11, 61.11 and 58.33%, respectively, while fungicides containing bupirimate (25% a.i), tebuconazole (43% a.i) and propineb (58% a.i) + cymoxanil (4.8% a.i) were more effective in the control of powdery mildew with potential to reduce the infection by 55.95, 39.12 and 20.84%, respectively.


INTRODUCTION
Apple production is new agro enterprise that was established in Uganda since 2000 in South-western highlands mainly in Kabale district (ICRAF, 2003).13 apple cultivars were introduced by NARO and ICRAF following farmers' demand for fruit trees.Two apple cultivars 'Anna and Golden Dorsett' were officially released in 2009 and are now widely grown in Uganda in the highlands of south western region as commercial varieties.The production of apples however is faced with a number of challenges among which diseases are one of the important constraints facing apple farmers.Apple scab and powdery mildew were identified as the most important diseases of apples in the country, with a potential to cause significant effects on the developing temperate fruit agro enterprise (Chemining et al., 2005).The apple scab pathogen Venturia inaequalis (Cooke) wint and the causal agent of powdery mildew (Podosphaera leucotricha (Ell.and Ev.) cause extensive crop losses in all apple production areas (Kerik, 2012;Brun et al., 2008).In the absence of proven diseases management technologies in Uganda, attempts to control these two diseases have been initiated through the use of fungicides and screening for tolerance among the introduced apple cultivars.However, no proper *Corresponding author.E-mail: abelarinaitwe@yahoo.com.Tel/Fax: +256 486-426 492.
information is currently available regarding appropriate fungicides for the management of apple scab and powdery mildew.Therefore, this study was established to document the status of these diseases in the country and to evaluate the effectiveness of the available fungicides in the control of the diseases at Bungongi substation, in Kabale located at 1830 m a.s.l so that information on effective fungicides could be released and recommended for farmers to use.

Disease initiation for fungicide effectiveness in disease management
One-year-old apple seedlings varieties of 'Anna' and 'dorsett' grafted on rootstocks of "bidden fielder" cultivar were used in this experiment.One set of the experimental material was defoliated to 100%, while other was left intact.Both sets of seedlings were inoculated with spores of powdery mildew and apple scab, prepared from infected leaves as described.Infected leaves were collected and immersed in distilled water for four hours.The leaves were then removed, the inoculum filtered, through a 0.2 mm sieve.The suspensions were adjusted with distilled water and standardized to 1 × 10 3 conidia per ml using a hemacytometer and used as a fine mist spray on the seedlings until run off.Inoculation was done in the evening to encourage spore germination and penetration.48 h after innoculation, seedlings were subjected to treatments of contact and systemic fungicides.To prevent interplot interference due to fungicide drift; a split plot design was adopted, where fungicides constituted the main plot and apple cultivars, the sub-plots.The main plots (fungicides) were separated from each other with polythene sheeting.For each combination cultivar by fungicide, three replicates were used.

Fungicide effectiveness in scab and powdery mildew management
Apple scab and powdery mildew incidence and severity were scored on the first ten leaves following inoculation, from 2009 to 2011.Data on the number of leaves with powdery leaf lesions and scab lesion were counted and used to calculate disease incidence, as a proportion of disease-infected leaves.The mean number of powdery mildew lesions and scab lesions per leaf was considered as the measure of disease severity that was used to compute the percentage leaf area affected (PLAA).Apple scab PLAA was estimated using a scale of 0 to 7, where; 1 =: 0% < percentage of scabbed leaf surface (sls)< 1%; 2 = 1%<sls<5 %; 3 = 5%<sls< 10%; 4 = 10< sls< 25%; 5 = 25%<sls<50%; 6 = 50%<sls<75%; 7 = 75%<sls (Parisi et al., 1993).Powdery mildew percentage of affected area was estimated visually on individual leaflets using a 0 to 4 scale, where 0: no symptoms; 1: 1 to 5%; 2: 5.1 to 20%; 3: 20.1 to 40%; 4: 40.1 to 100% (Kim et al., 2004).Data were recorded 10 days after fungicide sprays, and, every week subsequently for a period of 8 weeks per season.These data generated on the intensity of powdery mildew and scab was used to compute the area under disease progress curve (rAUDPC) using the procedure of Campbell and Madden (1990).

Prevalence of apple scab and powdery mildew in Southwestern districts of Uganda
Apple scab and powdery mildew incidence and severity were significantly different (P=0.05).The results of the current work showed a high incidence of apple scab and powdery mildew in the majority of districts surveyed.The high incidence and severity of the diseases in most of the districts surveyed is attributed to the lack of proper management practices and lack of knowledge on the appropriate fungicides to be used for the management of these diseases.Developing cultivars resistant to both scab and powdery mildew and use of synthetic fungicides are important for scab and powdery mildew management.In this study it was noted that districts which experience lower temperatures and higher rainfall, scab was a major problem whereas powdery mildew was less important in areas where temperatures are higher and rainfall is lower, powdery mildew is a major problem, and scab is less important.This is supported by report of (Blazek and Hlusickova, 2003).The study also showed no significant difference in infectivity of apple scab and powdery mildew between the two common apple varieties 'Anna' and 'Golden Dorsett' in all the district surveyed (Tables 1 and  2).Cultivars Anna and Golden Dorsett were equally affected by apple scab and powdery mildew in the surveyed district.

Conclusion
The interchangeable use of fungicides containing propineb (70%), Metalazyl 4% + Macozeb 64%),and bupirimate (25%) promotes the control of apple scab to levels below those causing economic damages.The products of Bupirimate (25%), Tebuconazole (43%) and Propineb (58%) + cymoxanil (4.8%) are recommended for the management powdery mildew.Incorporating cultural methods, such as defoliation and use of resistant varieties, could also help to reduce the severity of the investigated diseases.

Table 1 .
Prevalence of apple scab in the two varieties grown in south western Uganda.

Table 2 .
Prevalence of powdery mildew in the two apple varieties grown in Western Uganda.

Table 3 .
Average rAUDPC for powdery mildew and percentage reduction in severity for three seasons, as influenced by different fungicides in apple crops.
rAUDPC= relative area under disease progress curve, computed using the percentage of leaf area affected measured over a period of 8 weeks, for 2 consecutive experimental growing seasons.

Table 4 .
Average rAUDPC for apple scab and percentage reduction in the disease severity, for two seasons as influenced by different fu ngicides.