ISSN 1330-7142
UDK = 632.25:633.15(497.12)
OUTBREAK OF MAIZE DOWNY MILDEW (Sclerophthora macrospora) IN SLOVENIA IN 1999
S. Trdan, F. Celar
Scientific review
Pregledni znanstveni članak
SUMMARY
In 1999 a pronounced infection of maize with fungus Sclerophthora macrospora occurred in Slovenia, especially in its north-eastern and eastern regions. Symptoms, typical for infection with this fungus, were found on app. 1800 ha, which means on about 2.5 % of the acreage on which maize is grown. The infected plants were found only on those fields, or parts of those fields, which were flooded for some time soon after sowing or emergence of the crop. The percentage of the plants infected by the fungus was between 5 and 90 %, depending on the type of soil. This article presents different disease causing fungi, their taxonomy, and deals in detail with the species Sclerophthora macrospora, the only disease causing organism under our growing conditions. The article deals with hosts of the fungus, typical symptoms of the disease on maize (“crazy top”), pathogen biology, the mode of infection, its survival under unfavorable conditions, and possible protection against the infection. The article also describes the actual situation in Slovenia, and adds a comment on possible regular occurrences of this disease in future.
Key-words: Sclerophthora macrospora, downy mildew, maize, Slovenia
1. INTRODUCTION
In Slovenia, protection of maize against fungal diseases is given the least attention among economically important field crops. There are several reasons to justify this situation, doubtlessly the rather pronounced tolerance of hybrids for most economically important disease-causing organisms and the acreage field structure in Slovenia (very small fields), which make the spraying with fungicides before the significant occurrence of the disease-causing organisms rather difficult if not impossible, can be considered the main ones.
The narrow crop rotations, badly maintained drainage systems, the fact that the exchange of goods on the market is faster and more intense every year and so the entrance of non approved seed material into our country becomes easier, combined with new weather conditions which are getting more and more unpredictable can lead to pronounced infection of maize with some fungal diseases, which used to be of minor importance or have even not been spotted in Slovenia till now.
Among the fungal diseases, which have recently become economically important, and among which we can expose the northern leaf blight (Setosphaeria turcica (Luttrell) K. J. Leonard & E. G. Suggs) as well as the diseases of ear and stem caused by the infection with various Fusarium species, a new one occured in 1999, caused by the fungus Sclerophthora macrospora (Sacc.) Thirumalachar, C. G. Shaw & Narasimhan.
2. DOWNY MILDEWS ON MAIZE
America, and in warmer areas in North America, South Africa and Australia. Although none of them originally infected maize, because they emerged only on wild growing grasses, they can easily infect this crop, due to their quick adjustment to new hosts (Dickson, 1956; Frederiksen and Renfro, 1977; Cassini and Cotti, 1979).
The species Sclerophthora macrospora (Sacc.) Thirumalachar, C. G. Shaw & Narasimhan was first detected in Italy in 1902. Among all eight species of downy mildews on maize only three are economically important in the areas with moderate climate, in Europe, however, the only such organism is Sclerophthora macrospora. The species Sclerospora graminicola Sacc. & Schröt. is quite common in Central and Southern Europe, but mostly on economically unimportant hosts such as the grasses of the genera Setaria and Panicum and on some other wild growing species. Most recently (1961) another downy mildew has been discovered in the moderate areas of the USA. For the time being it is among the most important diseases, especially on sorghum and is being caused by Sclerospora sorghi Weston & Uppal (Frederiksen and Renfro, 1977; Gjaerum et al., 1986).
3. TAXONOMY
The latest systematics classifies downy mildews on maize in the order of Sclerosporales. The genus Sclerophthora belongs to the Verrucalvaceae family, where haustoria are not known. Zoosporangia are formed directly at the end of the hyphae or the asexual phase is not known. The fungus Sclerophthora macrospora is, according to its sexual stadium, a relatively closer to the representatives of the Sclerospora genus, while according to the sporangia (asexual) phase, however, it is closer to the Phytophthora genus (sporangia germinate indirectly with zoospores). The main difference between the genera Sclerophthora and Sclerospora is the formation of zoosporangia, the representatives of the former form them directly on the hyphae, while the representatives of the latter form them on the branched conidiophores.
The representatives of various genera occur on geographically separate areas: Sclerospora graminicola and Sclerophthora macrospora mainly in areas with moderate climate, the conidia forming species (Peronosclerospora) mainly in areas with tropical and semi-tropical climate (Frederiksen and Renfro, 1977; Cassini and Cotti, 1979; Brandenburger, 1985; Gjaerum et al.,1986; Hawksworth et al., 1995).
4. SCLEROPHTHORA MACROSPORA THIRUMALACHAR, C. G. SHAW & NARASIMHAN
4.1. The situation in Slovenia
The first serious infections of maize with Sclerophthora macrospora in Slovenia were observed in July 1999, i. e. in a year with an extremely abundant rainfall. The infected plants were found on the fields, which were temporary inundated in spring. Such locations were especially common in the well developed north-eastern part of the country (along the rivers Dravinja and Ščavnica, near Polskava and Pesnica, Slovenske Konjice, Velenje, Polzela, as well as in the region of Pomurje) and in the eastern part of Slovenia (along the Sotla river, near Breţice and on Imensko polje). The infected plants were also found on some other locations but the infection was much less common (near Komenda, Cerklje, Kranj and Škofja Loka) (Fig. 1). The most typical symptom - the “crazy top” - warned the experts about the downy mildew infection. But due to its periodical and scarce occurrence the difficulties in determing the pathogen were pronounced.
The intensity of the infection with the fungus Sclerophthora macrospora on the fields which were inundated after the sowing or at the time of emergence of the crop varied a lot. The symptoms of the infection were different and the ratio of the infected plants varied from 5 to 90 %. The infected plants were found on approximately 1800 ha (Agric. Ext. Serv. RS, 1999). On parts of the fields, which were not inundated the infection did not take place, which speaks for the importance of water presence for the germination of oospores.
Figure 1. Locations in Slovenia where Sclerophthora macrospora occurred on maize in 1999 Slika 1. Mjesta u Sloveniji gdje se Sclerophthora macrospora pojavila na kukuruzu u 1999.god.
4.2. Significance and host plants
Sclerophthora macrospora is the only downy mildew pathogen on maize under growing conditions in Slovenia. In 1959 it has been found in Našice (Croatia) for the first time. Later on, it was also found near Nova Gorica and Ptuj (Slovenia) where the pathogen was determined on the basis of the symptoms it causes on maize (Many authors, 1983). In Croatia, the significant occurrences of Sclerophthora macrospora were reported in 1975, 1989 and particularly in 1999 (Čizmić et al., 2000). In the USA it is generally considered - as in many other European contries - as an economically less important disease (Smiljaković and Penčić, 1976; Ullstrup, 1978; Zogg and Winter, 1978; Jons, 1980; Gjaerum et al., 1986; Zwatz, 1986; Winter and Menzi, 1991).
In Europe, the fungus Sclerophthora macrospora is present only to a limited extent. Namely, it can be found in lower acreages with insufficient drainage, next to streams, rivers, drainage canals. Therefore, pronounced infections on maize can be expected after heavy rains where the fields are inundated (Gjaerum et al., 1986).
The fungus was found on more than 140 grass species (Poaceae family) belonging to 46 genera, 22 of these occurring also in Slovenia (Agropyron, Agrostis, Avena, Bromus, Digitaria, Eleusine, Elymus, Eragrostis, Festuca, Hordeum, Lolium, Muhlenbergia, Panicum, Phalaris, Phleum, Poa, Secale, Setaria, Sorghum, Sporobolus, Triticum and Zea) (Gjaerum, 1986; Farr et al.,1989; Martinčič et al., 1999).
4.3. The symptoms on maize
4.4. Biology, mode of infection and survival in unfavorable conditions
Theoretically, young maize plants are exposed to infections through seeds, air or soil. Sexual spores (resting spores or oospores) of the Sclerophthtora macrospora species are hialine to yellowish, lemon-shaped, contain more than one nucleus, relatively large and have a thick wall. The species determination is based on the oospore type (its shape and size). In the case of Sclerophthora macrospora the size of the oospore is 60 to 65 µm, which makes them the largest among all the species of downy mildew on maize. They germinate and form lemon-shaped protosporangia (asexual spores), from these biciliate zoospores are released. The germination of resting spores is possible only after a period of dormance, no shorter than eight weeks. Zoospores infect the plant only if the ground was saturated with water for at least 24 to 48 hours. The roots attract zoospores chemotactically, they penetrate into the plant behind the root cap. After systemic infection rare sporangiophores grow out of the stomata. On them lemon-shaped sporangia are formed, which are small in size and measure 60-70 x 38-52 µm. They are extremely difficult to find on maize, thus making the determination of the pathogen more complicated.
The fungus survives in the form of oospores on the remaining parts of the infected maize plant or on wild growing grasses, in the form of latent mycelium, however, it remains in the infected kernels, too. Oospores retain their germination faculty up to 14 years in dry conditions, and up to 2 years in soil (Ullstrup, 1962, 1978; Frederiksen and Renfro, 1977; Raghavendra and Safeeulla, 1977; Cassini and Cotti, 1979; Gjaerum et al., 1986; Nyvall, 1989). Although the transmission of the downy mildew through infected kernels is relatively rare, utmost attention is needed to notice the possible occurrence of this disease, especially in conditions such in Slovenia and Croatia. Namely such an occurrence is most probable in those areas where the downy mildew appears only periodically, and is thus of a minor economical importance. Pathogen can also survive in the infected kernels, formed though rarely and in small number on infected plants. The kernels can remain in the ground till the next growing season and are thus, according to some references, the primary inoculum for new infections (Cassini and Cotti, 1979).
4.5. Control
Maize is sensitive to the infection with Sclerophthora macrospora only during the period of the first three to six weeks after sowing, until the plants reach the height of 15 cm and develop more than 4 to 5 leaves. Then the plants become more and more tolerant to the infection and later on even they seem to be resistant. The optimal temperatures for the infection are from 12 to 16 oC (Ullstrup, 1978; Many authors, 1983).
Among the measures to reduce the loss of crops because of these infections, the adequate drainage of the watersurplus from the fields in question is. Quick germination of seeds and quick growth at the beginning of growing period, which include adequate preparations of the fields as well as balanced fertilization, together with optimal environmental factors all help to reduce the loss of the crops.
Removing wild growing hosts is also considered as one of the important control measures. According to the biology of the fungus and taking into consideration the number of hosts, removing of the infected maize plants plays a secondary role.
With the intention of preventing the infections through the affected kernels, it is important to use healthy seed material only. Systematic infections of the plants with mycelium through the infected kernels can be prevented or at least reduced by drying the kernels to less than 10 % humidity, and not to sow them for some months after they have been stored. There are some indications, that treating the kernels with hot water and exposing them to the sun weakens the vigor of the mycelium inside the seed, whereas others claim that such treatments have no effect on the pathogen. In any case it is surprising how sensitive some of the hybrids are to the infections with some downy mildews, which shows a serious potential problem of modern maize production. Despite this, it was discovered in the USA by the end of the 70’s, that the production of maize drops only if 30 % or more plants are infected with Sclerophthora macrospora (Cassini and Cotti, 1979; Ullstrup, 1978; Graves et al.,1980).
In the areas where the disease occurs regularly and to a greater extent, the use of fungicides is economically justified. According to different authors, the successes in controling this disease varies quite a bit. Two ways of chemical control can be used: one is disinfecting the seed material, and the other spraying with fungicide during the period when the plants are susceptible to infection (Cassini and Cotti, 1979).
4.6. A commentary toward the situation in Slovenia and Croatia
that, a great number of farmers will experience the loss in this year crops because of the infection (less dry mass in the silage, because the ears have not formed). This reminds us always to be prepared. A constant monitoring of the seed origin and its quality is essential. The tolerance or resistance of maize cultivars to various fungal diseases is important, maintaining of efficient drainage systems, and environment friendly machine cultivation of the fields are crucial to make the soil less tough and more permeable to water, enabling a better natural drainage. The infections which occur through the roots by zoospores are only possible if the previous germination of oospores in the soil saturated with water has taken place.
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Trdan, S., Celar, F. (1999): The first and mass occurence of the fungus Sclerophthora macrospora (Sacc.) Thirumalachar, C. G. Shaw & Narasimhan on maize in Slovenia.- Res. Rep., Biotech. Fac., Univ. Ljubl., Agric. issue, 73(2): 271-280.
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U 1999. godini u Sloveniji su, osobito u sjeveroistočnim i istočnim krajevima, utvrđene jake zaraze kukuruza s gljivom Sclerophthora macrospora. Tipični simptomi bolesti utvrđeni su na otprilike 1800 ha, što znači na 2,5% svih njiva posijanih kukuruzom. Zaražene biljke nađene su samo na zemljištima ili njihovim dijelovima, gdje se brzo po sjetvi ili nakom nicanja zadržavala voda, a udio zaraženosti kretao se, u zavisnosti o svojstvima zemljišta, od 5 do 90%. U radu su prikazane različite gljive, koje uzrokuju plamenjaču kukuruza, te njihova taksonomija. Detaljnije je opisana vrsta Sclerophthora macrospora, koja jedina u slovenskim uvjetima prouzročuje spomenutu plamenjaču. Prikazani su domaćini gljive, tipični simptomi bolesti na kukuruzu, stadiji razvoja patogena, način infekcije, način kako preživljava u nepovoljnim uvjetima i moguća zaštita kukuruza. Prikazano je stvarno stanje u Sloveniji i dodan komentar o mogućem redovnom pojavljivanju te bolesti u narednim godinama.
