## 1 Introduction  
Plant fungal diseases are commonly controlled by applying chemical synthetic fungicides during farming or in packing houses. Alternative pest control methods are perceived as more environment-friendly and consumer-oriented, and should respond more adequately to the expectations of sustainable agriculture. However, compared to the number of active ingredients included in conventional chemical pesticides worldwide, the number of alternative methods currently licensed for disease control remains low.  
_Botrytis cinerea_ and _Penicillium expansum_ are ubiquitous plant-pathogenic fungi whose conidia are mainly airborne, but also dispersed by water, animals, and humans [1]. _B. cinerea_ can develop at cold storage temperatures and the disease spreads through rotted fruit contact [1]. _P. expansum_ causes one of the most common post-harvest rots of apple [1]. Its significance has increased in recent years because it produces patulin, a mycotoxin found in rotted fruit that increases fungal virulence [2,3] and is the major cause of patulin contamination of fruit juices, compotes, and other apple-processed products [4]. In addition to Pomaaceae, _P. expansum_ is a saprophyte of other crops such as citrus fruit and cherries; patulin has been detected in bananas, peaches, apricots, grapes, grape juice, and wine. In Europe, patulin has been detected only in apple samples such as compotes and cooked fruit (1 \(\mathrm{\SIUnitSymbolMicro g}\)/kg), fresh alcoholic beverages (0.12 \(\mathrm{\SIUnitSymbolMicro g}\)/kg) and fruit (0.04 \(\mathrm{\SIUnitSymbolMicro g}\)/kg) [4].  
Lactoperoxidase (E.C.1.11.1.7; LPO) is an enzyme that occurs naturally in bovine milk and is closely related to human haem-peroxidase found in exocrine secretions such as saliva, tears, airway mucosa, or intestinal secretions. In bovine milk, LPO exerts its antibacterial activity through oxidants produced in the presence of cofactors, i.e., hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) and the thiocyanate ion (SCN\({}^{-}\)) present at variable levels depending on what the cows are fed, and the iodide ion (I\({}^{-}\)) found at very low levels in bovine milk [5, 6, 7, 8]. The oxidants derived from SCN\({}^{-}\) and I\({}^{-}\) are hypothiocyanate (OSCN\({}^{-}\)) ions and hypoiodite (OI\({}^{-}\)) ions/iodine (I\({}_{2}\)), respectively [9,10]. These oxidative products have inhibitory properties against pathogenic bacteria, fungi, and viruses [8,11, 12, 13]. Many in vitro or in vivo applications of the LPO system have been tested. This includes the preservation of raw and pasteurized milk, storage of cosmetics, and oral care products like moisturizing gel and toothpaste. Furthermore, veterinary products, preservation of foodstuffs, or plant protection have also been tested [11,14, 15, 16, 17]. However, practical applications of the LPO system are limited. As the catalytic reactions are complex, various factors can dramatically reduce antimicrobial activity, and the transfer from in vitro to in vivo conditions has led to inconclusive data [12,18, 19, 20, 21]. Apart from the substrates of the LPO system, the presence of other compounds in the environment of the protein may also significantly influence its efficiency. The objective of this study is to investigate the factors that affect overall enzyme efficiency to provide information necessary for the establishment of optimal conditions for the functioning of the biological activity of the LPO system and help industrials and farmers in using this natural preservation system.