Strawberry (Fragaria × ananassa Duch.) is affected by several leaf diseases and the common leaf spot (CLS) caused by Mycosphaerella fragariae (Tul. & C. Tul.) Lindau is one of the most economically important. Control is mostly performed with synthetic fungicides despite the risk of damage to health of farmers, consumers, and the environment that they may cause if improperly used. Previous studies showed that extracts of turmeric (Curcuma longa), garlic (Allium sativum), rue (Ruta graveolens), and wormwood (Artemisia absinthium) had fungicide effect (11. Mello APOA, Zacharias MB. Efeito de extrato vegetal no crescimento de Colletotrichum acutatum do morangueiro. Rev. Ciên. Tecnol. Ambiente 2019; 9(1): e09151.,22. Balbi-Peña MI, Becker A, Stangarlinb JR, Franzener G, Lopes MC, Schwan-Estrada KRF. Controle de Alternaria solani em tomateiro por extratos de Curcuma longa e curcumina - II. Avaliação in vivo. Fitopatol. Bras. 2006; 31(4): 401-404.) which may reduce or even substitute synthetic fungicides.

Looking for providing alternatives mostly to organic growers, the field efficiency of the extracts and plant oils for the CLS and their effects on organic strawberry fruit production was evaluated.

The experiments were carried out at Universidade Estadual de Londrina (Londrina-PR, Brazil) (23°19'44.5"S; 51°12'17.3"W; 585 m) for two cycles (2013 and 2015) in bed (1.2 m wide) with two cultivation lines (0.30 x 0.30 m) (experimental unit: 2.4 m x 1.2 m). Camarosa (transplanted on August 7, 2013) and Albion (transplanted on August 17, 2015) cultivars were used.

The evaluations were made with the natural incidence of the CLS. For disease severity, four central plants of each plot were used. In each plant, two leaves were randomly evaluated, and each of the leaves (average of the three leaflets) received a value according to a diagrammatic scale (33. Mazaro SM, Gouvea A, Mio LLM, Deschamps C, Biasi LA, Citadin I. Escala diagramática para avaliação da severidade da mancha-de-micosferela em morangueiro. Ciên. Rural. 2006; 46(2): 648-652.). Then the area under the disease progress curve (AUDPC) was calculated.

For the first cycle, the treatments were: rosemary extract (Rosmarinus officinalis L.) (10 %), garlic extract (0.01 %), rue extract (10 %), Bordeaux mixture (copper sulfate, hydrated lime and water in proportion 1:1:100) (1 %), turmeric extract (1 %), cottonseed oil (1.5 %), canola oil (1 %), and water (control). For the second cycle, the treatments were: rosemary extract (20 %), rue extract a 20 %, and turmeric extract (2 %), canola oil (2 %), and water (control).

For the rosemary and rue extracts, 50g of leaves were separately crushed and blended with 100 mL of distilled water for 10 minutes. From these crude liquid extracts, aqueous extracts of each plant with 10 and 20 % concentrations were prepared for the first and second cycles, respectively (44. Röder, C. Controle Alternativo de Podridões na Cultura do Morango com Tratamento em Pré-Colheita. Dissertação de Mestrado em Agronomia. Universidade Estadual do Oeste do Paraná, Marechal Cândido Rondon, Brasil. 2006. 120 p.).

The garlic extract was prepared by crushing 10g of garlic for 1 L of water and left to rest for twelve days. After this period, the extract was diluted to 1 % with water.

The turmeric extract at 1 % was prepared with crushed rhizomes in distilled water.

For the oil products, canola and cottonseed oils were emulsified in water at the desired concentrations [cottonseed (1.5 %); canola (1 and 2 %), first and second cycles, respectively] adding neutral detergent (1.0 %).

The strawberries were harvested twice a week. The fruits harvested were from the plants that were used for the disease evaluations (four plants per plot). Defective plants and diseased fruits were discarded.

A completely randomized block design with five replications was used. The data were transformed into the square root of (x + 1), and an analysis of variance was performed. AUDPC and fruit production means were compared by the Scott-Knott test for first cycle and Tukey test were used for second cycle, both at 5 % probability.

In the first cycle, canola oil and the extracts of rue and rosemary showed lower AUDPC values ​​than the control (Table 1). In a previous study, the 10 % rosemary extract provided satisfactory control of the mycelium growth of Glomerella cingulata (Stoneman) Spauld. & H. Schrenk, (1903) and Colletotrichum gloeosporioides (Penz.) (55. Rozwalka LC, Da Costa MLRZ, De Mio LLM, Nakashima T. Extratos, decoctos e óleos essenciais de plantas medicinais e aromáticas na inibição de Glomerella cingulata e Colletotrichum gloeosporioides de frutos de goiaba. Ciên. Rural. 2008; 38(2): 301-307.). Canola oil controlled powdery mildew, Sphaerotheca pannosa var. rosae, on grapevine (66. Azam MGN, Gurr GM, Magarey PA. Efficacy of a compound based on canola oil as a fungicide for control of grapevine powdery mildew caused by Uncinula necator. Australas. Plant Pathol. 1998; 27: 116-118.), and linseed and rapeseed oils were successfully used for Uromyces appendiculatus control (77. Breiing V, Hillmer J, Schmidt C, Petry M, Behrends B, Steiner U et al. Fungicidal efficacy of drying plant oils in green beans against bean rust (Uromyces appendiculatus). Plants. 2021; 10(1): 143.). Although the lower AUDPC value presented by the turmeric extract was not significantly different from the value in the control, this extract provided a reduction in severity at 100 days after transplantation. In previous studies, Alternaria solani and Botrytis cinerea was observed to be controlled by curcumin and turmeric extract (22. Balbi-Peña MI, Becker A, Stangarlinb JR, Franzener G, Lopes MC, Schwan-Estrada KRF. Controle de Alternaria solani em tomateiro por extratos de Curcuma longa e curcumina - II. Avaliação in vivo. Fitopatol. Bras. 2006; 31(4): 401-404.,88. Chenyan H, Kai K, Wanling B, Wei S, Yongsheng L, Danfeng Z. Curcumin induces oxidative stress in Botrytis cinerea, resulting in a reduction in gray mold decay in kiwifruit. J. Agric. Food Chem. 2019; 67(28): 7968-7976.).

In the second cycle, the values of the CLS severity were higher than those observed during the first year (Table 1) since the plants were in their second production cycle. For both the first and the second cycles, there were reductions in severity between an evaluation date and the next one. This can be explained by the fact that the plant continues emitting new leaves. The treatments with rosemary (20 %) and canola oil (2 %) showed lower severity means than the control on three of the five evaluation dates. However, the AUDPC value was lower than the control only in the treatment with canola oil. Canola oil previously controlled powdery mildew in rose bush and rust (Puccinia arachidis Speg.) in peanuts (99. Chen W, Ko W. Flaxseed oil for control of peanut rust and its disease control mechanism. J. Phytopathol. 2015; 163: 271-278., 1010. Wojdyla AT. Effect of vegetable and mineral oils on the development of Sphaerotheca pannosa var. rosae - the causal agent of powdery mildew of rose. Bulg. J. Agric. Sci. 2015; 21(4): 855-862.).

Fruit production was lower in the plants treated with garlic or Bordeaux mixture and in the control than in the plants with other treatments (Figure 1). Although no control was provided by the treatments with cotton oil or turmeric extract (their AUDPC values were similar to those in the control), the fruit production in these treatments did not differ from that in the treatments with the canola oil, rosemary or rue, which did control the disease. Bordeaux mixture neither controlled the disease and nor increased production. The use of plant extracts and oils to control plant diseases, despite the great potential they have for being used in plant protection, is still an emerging topic approached in researches (1111. Ramos SMB, Almeida EFA, Rocha FDS, Fernandes MDFG, Santos EBD. Organic fertilization and alternative products in the control of powdery mildew. Ornam. Hortic. 2020; 26(1): 57-68.-1414. Fagundes MCP, Oliveira AFD, Carvalho VLD, Ramos JD, Santos VAD, Rufini JCM. Alternative control of plant pathogen fungi through ethanolic extracts of avocado seeds (Persea americana Mill.). Braz. Arch. Biol. Technol. 2018; 61: e18180052. ). Some extracts may even have an effect on inducing plant resistance, promoting systemic resistance against pathogens (1515. Persaud R, Khan A, Isaac WA, Ganpat W, Saravanakumar D. Plant extracts, bioagents and new generation fungicides in the control of rice sheath blight in Guyana. Crop Prot. 2019; 119: 30-37.). Therefore, future researches should study these biochemical chains that provide an effective control of diseases with a reduced use or none of chemical products.

In summary, canola oil (1 and 2 %) decreased the incidence of common leaf spot in both years. Rosemary and rue extracts controlled only when the disease severity was low. Cotton and canola oils and the aqueous extracts of rosemary, rue and turmeric provided an increase of strawberry fruits.