Exogenic, cobweb mycelium of powdery mildew on the upper leaf surface 

Context in source publication

Context 1

... plant diseases known as powdery mildews have been reported for a long time in the literature (Linnаeus, 1753, quoted by Belanger et al., 2002). The classical researches of this group of diseases started after publishing the fundamental works of Leveille (1851) on the taxonomy of powdery mildews (Braun et al., 2002). Evolutionary, this group of fungi invades new ecological niché and host plants (Hirata, 1966; Gorlenko, 1983; Amano, 1986). Their systematic structure is constantly complicating. New genera and species are appearing in the Family Erysiphaceae (Braun et al., 2002). This phe- nomenon can be explained with the adaptation to the constantly changing ecological conditions. For example, genus Leveillula originates from the genus Erysiphе , as the development of these fungi is connected with adapt- ing to hot and dry conditions. In connection with the environment the evolution is seen not only in the tel- eomorph but also in the anamorph-conidia stage. The latter (anamorph) has clearly expressed species specifi- city (Braun, 1980). The structure of the conidia appara- tus depends on the climate, where different fungal spe- cies develop. The single conidium coming through the stomata, are formed in fungal pathogens, living in are- as with dry and hot climate (Gorlenko, 1983). The ec- ological conditions where plant species grow can proba- bly explain the appearance of new phytopathogens from different genera of the family Erysiphaceae , adapted to live parasitically on new host plants. During the first half of 20 th century more than 200 fungal species of family Erysiphaceae were reported. They parasitize more than 1500 different plants – culti- vated and wild, field, fruit and wood species (Atanasoff, 1934). Fakirova (1991) described 104 species and 4 vari- eties that belong to 10 genera of the family Erysiphace- ae . 95 species out of 104, and 4 varieties are registered in Bulgaria, where 28 pathogens were reported for the first time in the country (Fakirova, 1991). The pathogenic fungi develop on 432 higher plants, and 201 of them are new host plants in Bulgaria. Probably, the great plant di- versity in Bulgaria, favorable ecological conditions and high variability of the pathogens cause peculiar “geo- graphical” specialization (Vasjagina et al., 1961). The aim of the study was to describe the symptoms and to identify the causal agent of the disease. The research was carried out during 2007-2010 at the Department of Phytopathology, at the Agricultural University of Plovdiv. Pathogen identification was done based on the disease symptoms and on morphological characteristics of teleomorph and anamorph “which in high degree describe the species specificity (specific characters) of the pathogen” (Braun, 1980; Gorlenko, 1983; Fakirova, 1991; Braun et al., 2002). The descrip- tion of symptoms was done on the naturally infected plants, from cotyledon stage to seed formation and rip- ening, and on artificially inoculated spinach plants. Following treatments were examined in a way to dis- cover the first appearance of symptoms, to follow the de- velopment of powdery mildew, as well as the formation of the pathogens structures (anamorph and teleomorph): • Field experiments in 2008 and 2009: spinach seeds were sown in areas (two replicates, 25m 2 each) in- fected with powdery mildew during the previous year (2007 or 2008), with extra introduced inoc- ulum from cut powdery mildew infected spinach leaves, flowering stalks and stems. • Micro experiments in laboratory (2008-2010): spinach seeds were sown in river sand and sterile peat mixture, 1:1 (company Diko-2004) in plastic containers, sized 50 x 30 x 20 cm, in four replicates. The substrate was inoculated by mixing with cut spinach leaves, flowering stalks and stems infected by powdery mildew. • Control plants: Spinach seeds were sown in a ster- ile peat and sand mixture, 1:1, without inoculum in- troduction in containers (in field and in laboratory). During April-May 2007 in the spinach ( Spinacia ol- eracea L.) crops plants with suppressed growth, small- er and degenerated young leaves were found. In a very short time the affected leaves become yellowish and wilt, and when high temperatures and draught occur in the following period, the plants dry out. The report- ed symptoms resemble physiological degeneration, but differ from them because they do not affect the whole crop. After careful examination, fine, exogenic, cob- web mycelium is found on the leaf blades (Figure 1). The mycelium can be clearly seen close to main veins where it becomes thicker and forms mycelium patch- es. On the leaf and flower stalks, and the plant stems the mycelium is white, glossy and fine at the beginning, and later becomes gray. The teleomorphs of the patho- gen are formed on the infected organs. Under the microscope ectophytic mycelia with ex- ogenic origin conidiophores is discovered. On the hypha, on the short conidiophores, chains of two ty- pes of conidia are formed: macro - and micro-conidia. With the development of the disease, mainly close to the leaf veins, the pathogens cleistothecia are formed. They are found as single structures or in groups, and have filiform appendages hooked at the end. A new type of powdery mildew on spinach caused by was reported in the literature (Park et al., 2008). This fungus is related to Leveillu- la spp., whose representatives have conidiophores of endogenic origin with one conidium on the top (type Pseudoidium ) (Fakirova, 1991; Braun et al., 2002). For this species teleomorph has not been discovered. The research shows that the observed symptoms in Bulgaria differ from the ones described in the literature (Park et al., 2008). There are also differences in mor- phological characteristics of the anamorph stage. The pathogen in our country forms short conidiophores with micro- and macro- conidia in chains, which be- long to type Euoidium (genus Sawadaea ) (Braun, 1981; Braun et al., 2002) (Figures 2, 3). The macro conidia have an elliptical to cylindrical form, sized 24.5-28.4 х 17.5μm, while the micro conid- ia are oval to elliptical, 10.4-14.1 х 7.7 μm. The teleomorphs are roundish, dark brown with straight appendages hooked at the end. They are formed under unfavorable conditions – dry weather and high temperatures (dry and hot weather). They contain 4-5- 8 asci (68.0 x 38.0 μm), with 4-5 to 6-8 elliptical as- cospores, 15.5-22.0 х 11.0-17.5 μm. The observed symptoms and morphological charac- teristics correspond to the described ones for the path- ogens of the genus Euoidium (genus Sawadaea ) with two spore types-macro- and micro- conidia. The spe- cies Sawadaeа bicornis (syn . Uncinula bicornis ) and Sawadаea tulasnei ( syn. Uncinula tulasnei ) are report- ed in the literature (Fakirova, 1991; Braun et al., 2002). Based on the research results it can be accepted that spinach powdery mildew that appeared in Bulgaria is caused by Uncinula spp. ( Sawadaea spp.), an anamorph type Euoidium , subspecies spinaciae . According to Mori et al. (2000) and Takamatsu et al. (1998) the ge- nus Sawadaea differs from genera Cystotheca and Po- dosphaera , and is characterized with poliascal ascoma- ta with clearly formed appendages, with uncinate-circi- nate apex. It also has a different type of a conidial stage, belonging to genus Oidium subgen. Octagoidium . The observations on the development of the teleo- morph stage show that cleistothecia formation devel- ops mainly in mycelia patches, close to main veins. They are formed in large numbers on the flower stalks, too. Formed asci and ripen ascospores are found at the end of spinach vegetation period. The results obtained in the field and micro experi- ments are also in agreement with these data. Under lab- oratory conditions the first symptoms are seen on coty- ledon leaves, which become yellowish, necrotic and fi- nally drop out. Typical symptoms are discovered at the stage of 2 nd to 3 rd true leaf. On a large scale powdery mildew mainly appears on young, still growing leaves, as well as on the flowering stems. Based on the research results, the following conclu- sions can be made: • Powdery mildew discovered on spinach ( Spina- cia oleracea L.) in Bulgaria is caused by Uncinu- la spp. ( Sawadaea spp.), Euoidium type an- amorph probably subspecies spinaciae . • The pathogen is preserved in plant debris in the soil as a teleomorph and perhaps in ...