Tree peonies belong to section Moudan of the genus Paeonia in the family Paeoniaceae. They are an important group of flowering plants referred to as “the king of flowers” as a result of their large flowers, gorgeous color, elegant shape, and rich fragrance. Nowadays, tree peonies have become an internationalized ornamental as well as oil plant and are widely cultivated in many countries (Cheng, 2007; Liu et al., 2018; Zhang et al., 2018). They are unique, traditional, precious plants in China and they have also been cultivated extensively in the United States, France, the Netherlands, and more (Zhao and Tao, 2011). However, gray mold invariably occurs on herbaceous peony plants, especially when grown in the greenhouse (Yang et al., 2017). The symptoms seriously affect the ornamental and commercial values of the plants.
Studies have reported that the main pathogens causing gray mold on tree peony are Botrytis paeoniae and B. cinerea (Hansen, 2009; Pfleger et al., 1998). Yang et al. (2017) confirmed that B. cinerea was the only pathogenic fungus causing tree peony gray mold in Luoyang through the identification of a large number of samples based on morphological characteristics and gene sequencing. It has been recorded that the pathogen can attack more than 200 plant species and results in considerable economic losses worldwide (Williamson et al., 2007). B. cinerea can attack different plant organs, including flowers, leaves, fruits, shoots, and soil storage organs. Both conidia and mycelium can serve as a primary inoculum, and they cause disease in many plants. In perennial plants, the dead flowers, leaves, and mummified fruit contain masses of mycelium that can produce conidia and initiate infection (Williamson et al., 2007), which leads to a difficulty in controlling the disease because of the pathogen’s diverse attacking modes and plenty of host plants (Williamson et al., 2007).
In recent years, more plastic greenhouses are widely used in peony-growing areas in China to advance florescence for viewing, especially in parks with peonies. However, this measure also increases the incidence of peony diseases, especially gray mold caused by Botrytis cinerea (Duan, 2009). A disease investigation carried out by Yang et al. (2017) in 2014–15 showed that gray mold is one of the major diseases that affects the normal growth and development of tree peony in Luoyang. This area is one of the main cultivation areas of tree peony; it has a long cultivation history, a large cultivated area, and various varieties in China. Another investigation conducted in 2014 on potted tree peonies grown in a greenhouse showed that gray mold is also the main disease in forcing culture of tree peonies (Yang et al., 2017). In general, the disease has become an important limiting factor of tree peony production and seriously affects the ornamental value of tree peonies both in the open air and in greenhouses.
At present, the effect of using resistant varieties to control the disease is still a long way to go because the genetic relationship among existing tree peony varieties is mostly unclear, which leads to low breeding efficiency and the new variety release rate (Guo et al., 2017; Guo et al., 2018). As a result of the lack of the gray mold-resistant germplasm resources, chemical control is still the main way to reduce the incidence of gray mold on major crops and other plants, although resistance is an inevitable result of evolution. However, there are few reports that document which chemical fungicides are effective in preventing and controlling the gray mold disease at different stages. In our study, five different classes of fungicides—anilinopyrimidine (e.g., pyrimethanil), carboxamide (e.g., boscalid), benzimidazole (e.g., carbendazim), dicarboximide (e.g., procymidone), and N-phenylcarbamate (e.g., diethofencarb)—were used to test the sensitivity of fungicides with different mechanisms of action in controlling the pathogen of gray mold on tree peony in vitro (Avenot and Michailides, 2010; Chapeland et al., 1999; Liu et al., 2016). In addition, the morphological effect of the fungicides on the growth and development of B. cinerea was evaluated, and its application methods were analyzed.
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