Biljna bolest pepelnica koju uzrokuje gljiva Microsphaera alphitoides, ima snažan utjecaj u procesu sušenja hrasta. Najveća opasnost prijeti poniku i pomlatku. Iskustva šumara s područja pridolaženja hrasta lužnjaka zadnjih desetljeća su da je pepelnica jedan od bitnih čimenika preživljenja hrastovog pomlatka, a time i obnove hrastovih šuma. Iako vrlo patogena gljiva, istraživanja njezinog utjecaja na razvoj i sušenje pomlatka hrasta koja bi opravdala sredstva uložena u zaštitu, relativno su malobrojna. Cilj istraživanja je odrediti stupanj ovisnosti visinskog i debljinskog prirasta te preživljenja ponika o intenzitetu zaraze pepelnicom. Pokus je postavljen po metodi slučajnogblok rasporeda u ograđenoj površini na području Uprave Šuma Podružnica Bjelovar. Pokus je po istoj metodi postavljen i u rasadnika Hrvatskog šumarskog instituta. Sakupljeni žirovi zaštićeni su fungicidima te posijani u redove u ograđenoj površini.
Na poniku zaštićenom fungicidima, u 10 sati izmjeren je intenzitet fotosinteze od 8,31 µmol CO2 m-2s-1,dok je u 14 sati izmjereno 6,76 µmol. Na zaraženom (nezaštićenom) poniku izmjereno je u 10 sati znatno manje, 5,38 µmol CO2m-2s-1odnosno u 14 sati 3,75 µmol. To rezultira znatno manjom prosječnom visinom netretiranog ponika pod zastorom krošanja (22,62 cm), dok je prosječna visina ponika tretiranog fungicidima (25,57 cm). Suprotno tomu, u rasadniku nezaštićene biljke imaju na kraju prve vegetacije srednju visinu 24,82 cm, dok zaštićene nešto manju 22,56 cm. Masovno opadanje lišća dogodilo se kod nezaštićenog ponika 12 dana ranije nego kod zaštićenog. Preživljenje nezaštićenog ponika u šumi na kraju prve godine je 78 %, dok je preživljenje zaštićenog 94 %. Preživljenje je u rasadniku približno jednako u oba tretmana. Kod nezaštićenog ponika preživjelo je 92 % biljaka, odnosno 95,5 % kod zaštićenog.Powdery mildew as plant disease caused by the fungus species Microsphaera alphitoidesGriff et Maubl., has major impact in process of oak trees dieback. Most threatened are seedlings and saplings. Experiences from professionals in last few decades proved that within the habitat of Pedunculate oak, powdery mildew is one of important factors that affects survival rate of oak’s saplings, therefore rejuvenation of oak forests. Although fungus highly pathogenic, the researches of its influence on development and dieback of saplings in order to justify the investments in protection are relatively few. The researches targeted to determine the level of dependence between the height increment and diameter increment in relation with the survival rate of seedlings and the intensity of contamination caused by mildew, are set on research plots according to the block distribution in fenced surfaces, under the Forest District Bjelovar of the Croatian Forests Ltd. The experiment is set on the same model also in the Croatian Forest Research Institute’s nursery. The acorns collected were treated with fungicides and sawn in rows in fenced surface. In experiment two variants were tested in four repetitions; seedlings treated with anti-mildew fungicides and seedlings without any protection. After germination in one-month intervals the plots were cleaned from weed and the height increment was measured, whilst based on the surface of leaves covered with mycelia the infection caused by powdery mildew was estimated on 50 leaves sample in every plot. Fungicides were applied every 15–20 days depending of weather conditions. For the protection three fungicides were used: Artea 330 EC (0.5 l / ha), Anvil 5 SC (0.5 l /ha) and Punch 10EW (0.3 l / ha) all with 250 l/ha of water and 0.2 % Sandovit surfactant. Fungicides were used in forest nurseries seven times and five times in forest. The intensity of photosynthesis was measured on infected and non-infected leaves with portable gas-exchange system Li-Cor LI- 6400. On seedlings treated with fungicides at 10.00 hours was measured 8,31 µmol of CO2m-2s-1while at 14.00 hours was measured 6,76 CO2m-2s-1. On infected seedlings at 10.00 hours was measured significantly lesser value 5,38 µmol of CO2m-2s-1while at 14.00 hours 3,75 CO2m-2s-1. The research results show the direct relationship between the infestation by powdery mildew and the intensity of photosynthesis with the height increment and the survival rate of seedlings. This relationship is particularly denoted inside the forest, under the canopy where average height of seedlings (Graf 4) is significantly lesser at the infected seedlings (22,62 cm) compared with the seedlings treated by fungicides (25,57 cm). At the infected seedlings appeared massive defoliation 12 days earlier than at the uninfected. The survival rate of infected seedlings (Graf 7) in the forest at the end of the 1st year is 78 % while the survival of treated seedlings is 94 %. In opposition inside the nursery (Graf 5) the infected plants have average height of 24,82 cm at the end of first vegetation, while treated are relatively smaller 22,56 cm. The survival rate in the nursery (Graf 6) is approximately equal in both treatments, at the infected seedlings 92 % comparing to 95,5 % at the uninfected
