Field Efficacy of Mandipropamid for the Control of Potato Late Blight

Emil Rekanović1, Miloš Stepanović1, Milan Stević2, Ivana Potočnik1, Biljana Todorović1 and Svetalana Milijašević-Marčić1 1Institute of Pesticides and Environmental Protection, Laboratory of Applied Phytopathology, Banatska 31b, 11080 Belgrade, Serbia (emil.rekanovic@pesting.org.rs) 2University of Belgrade, Faculty of Agiculture, Institute of Phytomedicine, Nemanjina 6, 11080 Belgrade, Serbia Received: March 25, 2010 Accepted: April 8, 2010


INTRODUCTION
Potato late blight caused by Phytophthora infestans (Mont.) de Bary is the most important foliar and tuber disease of potato, both in field and under storage conditions (Fry and Goodwin, 1997). The incidence of a more aggressive strain of P. infestans, resistance of this pathogen to metalaxyl and the potential loss of some currently affective protective fungicides, contributed to the complexity of the issue (Kirk et al., 2000). The continuous search for new anti-oomycetic compounds, driven by resistance problems and the need for environmentally safe pesticides, suggested recently that mandipropamid may resolve the issue (Lamberth et al., 2006).
Mandipropamid is a new fungicide effective against foliar oomycete pathogens but it does not control Pythium spp. (Huggenberger et al., 2005;Cohen et al., 2007). Together with dimethomorph, flumorph, iprovalicarb and benthiavalicarb, it was classified into one group of carbixylic acid amide (CAA) fungicides, mainly because filed isolates of Plasmopara viticola, showed cross-resistance to all the members of the group (FRAC, 2005;Cohen et al., 2007;Gisi et al., 2007). The mode of action of CAA fungicides is not known. Biochemical studies with the mandelamide compound SX 623509 in mycelium of P. infestans suggested alternations in phospholipid biosynthesis, with inhibition of phosphatidylcholine (lecithin) biosynthesis as the main target (Griffiths at al., 2003;Cohen et al., 2007). The suggested inhibited enzyme was phosphocholinetransferase, the last enzyme in the Kennedy pathway (Griffiths at al., 2003;Cohen et al., 2007).
Mandipropamid is highly effective in preventing spore germination. It is also an inhibitor of mycelial growth and sporulation (Huggenberger et al., 2005). Being rapidly adsorbed to the wax layer of the plant surface, mandipropamid provides a rainfast and longlasting barrier to fungal diseases (Lamberth et al., 2008). Mandipropamid has a moderate amount of translaminar and acropetal systemicity, and is most effective when used as a protectant fungicide; however, it has some degree of post-infection activity (Stein and Kirk, 2003).
The objective of this study was to evaluate the efficacy of mandipropamid against Phytophthora infestans in commercial potato fields in Serbia.

MATERIAL AND METHODS
Field experiments were conducted to evaluate the efficacy of mandipropamid (250 g/l, Revus 250 SC, Syngenta AG) against potato late blight. Azoxystrobin (250 g/l, Quadris, Syngenta AG) served as the standard treatment. Application rates are listed in Tables  2 and 3 The tests were laid down as randomized plots (25 m 2 , 5.0 X 5.0 m), replicated 4 times with 90-100 plants/plot. Seed potato tubers (variety Desiree) were used in all the trials and sown into the soil between mid-April and early May.
Applications were made using a knapsack sprayer (Solo 425, Germany) to simulate practical applications by farmers (water volume: 600 liter/ ha). Initiation of application was generally adjusted to local practice in Serbia, starting from BBCH (Biologische Bundesanstalt, Bundessortenamt and Chemical Industry) 55-65 (pre-to full-flowering) until BBCH 70-75 (beginning of maturity) (Meier, 1997). Five foliar sprays were applied in all the trials at approxymately 6-12-day intervals in preventive programmes. Application details are listed in Table 1.
Disease incidence was evaluated approximately two weeks after the last fungicide application. In every assessment, 20 plants in the central part of each plot were evaluated. Disease percentage on plants was rated on a scale of 0 (no disease) to 5 (more than 50% infested) according to EPPO guideline PP 1/2(3) (EPPO, 1997a). Disease severity was evaluated using Townsend-Heuberger's formula (Puntner, 1981): Disease severity = Σ(scale x number of plants) x100 / 5x20 The data were analysed by one-way completely randomized ANOVA and means were compared according to Duncan's test (EPPO, 1997b;EPPO, 1997c).

RESULTS
In 2007, disease pressure in the trials was significant with 13.4-31.3% infected potato plants per plot in the untreated control. All the treatments were effective against the potato late blight pathogen and reduced disease severity significantly in all the experi-ments, compared with the untreated plots. Among the tested fungicides, the higher concentration of Revus 250 SC showed the highest efficacy in all trials (97.1 and 99.0%) ( Table 2). There was no significant difference between the efficacies of Revus 250 SC at both concentrations (96.3-93.8%) and standard fungicide Quadris (94.1 and 95.2%) ( Table 2).    Table 3). There was no significant difference between the efficacies of the tested fungicide Revus 250 SC (96.5-99.5%) and Quadris (94.6 and 95.5%) ( Table 3).

DISCUSSION
This study shows that the mandipropamid is highly effective against P. infestans, even under high disease pressure, confirming and extending the data obtained in previous trials conducted on potato in Israel (Cohen et al., 2007). The new products showed a remarkable activity against potato late blight ensuring longer intervals of protection during periods of high epidemic risk (6-12 days).
Similar trials carried out on several locations in Serbia, also showed that mandipropamid was the most effective in potato late blight control (Stević et al., 2007;Rekanović et al., 2008).
Whilst resistance to CAAs in P. viticola was detected in 1994, shortly after the introduction of dimethomorph in France, no resistance has been reported in filed isolates of P. infestans, even though dimethomorph has been in use for more than 15 years (Cohen et al., 2007;Gisi et al., 2007). The reasaons of this are lower fitness of resistant isolates P. infestans, different epidemiology of this two pathogens (oospores of P. viticola initiate epidemics in the spring, whereas P. infestans does not necessarily do so), and the fact that in P. viticola, resistance to CAAs in filed isolates was found to be controlled by recessive genes (Gisi et al., 2007). Recessive genes for resistance would need several sexual recombinations to be fixed in the population and expressed phenotypically, which in P. infestans may not happen frequently enough for establishment in the field (Cohen et al., 2007).
Sensitivity to the mandipropamid in P. infestans was measured for isolates collected between 1989 and 2002 in Israel prior to the commercial use of this fungicide (baseline sensitivity, 44 isolates), and from mandiprop-amid-treated (25 isolates) and untreated fields (215 isolates) in nine European countries and Israel between 2001 and 2005. All the isolates were sensitive to mandipropamide, with EC 50 values ranging from 0.02 to 2.98 μg/ml (Cohen et al., 2007). Although this experiment suggests that isolates resistant to CAA fungicides may not appear in field populations, suitable anti-resistance precautions should be taken, including continuous monitoring, preventive use of the products, limitation of the number of applications per season (maximum 50 % of the total number of intended applications for late blight control), and use of CAA products within a spray programme with fungicides of other chemical classes (Cohen et al., 2007;Anonymous, 2009