«Department of Plant Genetics, The Weizmann Institute of Science, Rehovot, Israel and “Eden” - Regional Experimental Station, Bet-Shean, Israel ...»
J. Phytopathology 133, 225-238 (1991)
© 1991 Paul Parey Scientific Publishers, Berlin and Hamburg
Department of Plant Genetics, The Weizmann Institute of Science, Rehovot,
Israel and “Eden” - Regional Experimental Station, Bet-Shean, Israel
Wind Dispersal of Alternaria alternata,
a Cause of Leaf Blight of Cotton
YOAV BASHAN*, HANNA LEVANONY and REUVEN OR
Authors’ addresses: Y. BASHAN, Department of Microbiology, The Center of Biological Research (CIB), La Paz, P.O. Box 128, B.C.S. Mexico 23000; H. LEVANONY, Department of Plant Genetics, The Weizmann Institute of Science, Rehovot 76100, Israel; R. OR, "Eden" Regional Experimental Station, M. P. Bet-Shean, Israel.
With 6 figures Received May 29, 1990; accepted December 14, 1990 Abstract Introducing Alternaria alternata, the cause of blight disease of cotton plants, into a field of young healthy plants growing in rows cross-wind, yielded disease foci which were spread downwind up to 7 m from the infection sources. Only light disease incidence was found in the remainder of the field. When the disease was introduced into a field of mature cotton plants grown in rows cross-wind, randomly scattered disease foci occurred. In mature plantations where rows were parallel to the average wind direction, only limited size disease foci developed downwind, up to 16 m from the source. These foci did not developed further during the season. The number of air-borne spores of A. alternata was significantly increased by the presence of diseased cotton plants, being highest close to the diseased plants. The spores were transferred to a distance of at least 20 m. However, the number of air-borne spores significantly decreased 6 m from the infection source. Periodical trapping of air-borne spores of A. alternata in a cotton growing region for 2 years, revealed that their air dispersal is local, probably at the field level. A. alternata air-borne spores were also trapped in rather low numbers regardless of the presence of infected cotton plants. However, the number of the airborne spores trapped was dependent mainly on the average wind direction and on the Alternaria blight epidemics occurring in the fields twice a year. It is suggested that A. alternata spores are transferred by wind for short distances but are constantly present in small numbers in the atmosphere throughout the whole year. The two peaks recorded for the number of spores present in the air above cotton crops correlate with the annual two outbreaks of Alternaria blight epidemics. In addition, both wind and plant row direction affect disease development in the fields.
* Corresponding author.
Wenn Alternaria alternata, die Ursache einer Blattfäule von Baumwollpflanzen, in einem Feld eingeführt worden war, wo junge, gesunde Pflanzen in Reihen im Seitenwind wuchsen, entwickelten sich Krankheitsherde, die sich bis zu 7 m von der Infektionsquelle in Windrichtung ausdehnten. Im Feld wurde sonst nur ein leichter Krankheitsbefall festgestellt. Unter ähnlichen Bedingungen, aber mit volt entwickelten Pflanzen, wurden nur vereinzelte, verstreute Krankheitsherde beobachtet. In volt entwickelten Beständen, wo die Pflanzenreihen parallel zu der allgemeinen Windrichtung standen, entwickelten sich Krankheitsherde bis zu 16 m von der Infektionsquelle in Windrichtung. Diese Krankheitsherde entwickelten sich w ährend der Saison nicht weiter. Die Anzahl der in der Luft vorhandenen Sporen von A. alternata wurde durch das Vorhandensein von erkrankten Pflanzen signifikant erhöht, and die höchste Anzahl an Sporen wurde in unmittelbarer Nähe von erkrankten Pflanzen ermittelt. Die Sporen wurden mindestens 20 m weit von der Quelle verbreitet. Die Anzahl der Sporen verringerte sick jedoch signifikant 6 m von der Quelle. Das unregelmäßige Erfassen des Sporenfluges von A. alternata in einem Baumwollanbaugebiet über 2 Jahre zeigte, dal deren Luftverbreitung nur lokal ist, wahrscheinlich innerhalb eines Feldes. A. alternata-Sporen wurden auch ohne das Vorhandensein von infizierten Baumwollpflanzen in der Luft erfaßt, obwohl nur in verhältnismäßig niedriger Anzahl. Die Anzahl der in der Luft erfaßten Sporen war jedoch haupts ächlich von der durchschnittlichen Windrichtung and der Alternaria-Fäuleepidemien, die in den Feldern zweimal im Jahre vorkommen, abhängig. Es wird vorgeschlagen, daß A. alternata-Sporen fiber kurze Strecken mit dem Wind verbreitet werden, sie Bind jedoch in niedriger Anzahl das ganze jahr in der Luft vorhanden. Die zwei Spitzen der Anzahl der in der Luft vorhandenen Sporen korrelierten mit den zweimal im jahre auftretenden Alternaria-Fäuleepidemien. Außerdern beeinflußte sowohl die Windals auch die Pflanzenreihenrichtung die Krankheitsentwicklung in den Feldern.
Air-borne spores of many plant pathogens have been attributed as causal agents of epidemics in many crops worldwide (AYLOR 1986, GREGORY 1968, R AO and MALLAIAH 1988). Aerial surveys to predict possible epidemic outbreaks using passive (sticky surfaces, solid medium surface) (ATLURI et al. 1988, BANERJEE et al.
1987) or active (volumetric) (EVERSMEYER et al. 1973, MCCRACKEN 1989) traps are a common practice.
Usually data, relating to air-borne spores in epidemiological studies of plant diseases or in studies of allergies caused by fungal spores, are collected from single traps located at one or several sampling sites in an area, and are considered sufficient to draw reliable conclusions (EVERSMEYER and KRAMER 1987b, GREGORY 1968). A significant amount of data relating to air-borne spores has been accumulated in relation to plant diseases yielding several general conclusions: (i) there are significant variations in trapping air-borne spores of fungi directly related to the biometeorological factors prevailing during sampling time (LONG and KRAMER 1972, LYON et al. 1984 b), (ii) there are unique patterns of aerial distribution mainly at the fungal genus level (EVERSMEYER and KRAMER 1987 a, KRAMER and EVERSMEYER 1984, KRAMER et al. 1963), (iii) spore dispersal via air currents can be considered in terms of air dispersal of small particles (BURROWS 1983, 1988, GREGORY and STEDMAN 1953) regardless of viability (FAULKNER and COLHOUN 1976, WALE and COLHOUN 1979), (iv) wind can be considered as the main factor affecting aerial dispersal of propagules (VOLCANI 1969, WAGGONER
1973) and (v) temperature as well as water, either by precipitation or irrigation, is of utmost importance to the growth of fungi and directly affects the number of Wind Dispersal of Alternaria alternata, a Cause of Leaf Blight of Cotton spores produced and released into the air (K RAMER et al. 1963, LYON et al. 1984 b).
The present study has thus concentrated on the effect of wind on aerial distribution and initiation of disease epidemics of A. alternata, one of the causal agents of Alternaria blight of cotton.
Alternaria blight disease is the most severe leaf disease of cotton plants in Israel (HADAS and JACOBY 1981) and was attributed to the species A. macrospora affecting mainly G. barbadense plants of cv. Pima and its derivatives (BASHAN 1984, 1986 b, BASHAN and LEVANONY 1987, EBBELS 1980, HADAS and JACOBY 1981). This species has the ability of being transferred within the growing season by various biotic and abiotic vectors (BASHAN 1986 a). Recently, strains belonging to the A. alternate complex of species (SIMMONS 1967) caused also Alternaria blight in plants of G. hirsutum cv. Acala (LEVANONY et al. 1988) and of G. arboreum (SINGH et al. 1984).
Disease outbreaks in Bet-Sheen valley occur annually twice a season: first in the cotyledon and young seedling stage in April- May and second, in September-October late in the cotton growing season, just before the commercial harvest. Although chemically treated by frequent aircraft sprayings, effective disease control or increase in yield is rarely achieved. Moreover, the most effective fungicide (fentin-acetate, formulated product, Bedilan 60 w.p.) is phytotoxic to cotton seedlings (ZACKS 1990). A. macrospora can induce symptoms mainly in the cotton species G. barbadense (long fiber cotton). G. hirsutum (short fibre cotton) cv. Acala is almost resistant to this pathogen. On the other hand, A. alternata is capable of infecting both cotton species. However, it has a preference to attack G. hirsutum plants. Recently, it was proposed that A. macrospora together with A. alternata form a disease complex responsible for Alternaria blight disease in both cotton species (BASHAN et al. 1991).
Spores of Alternaria spp. are abundantly dispersed in the atmosphere and are used as a common model in allergic studies (DURHAM 1944, 1946). Abundance of Alternaria spores in the air was not correlated with plant vegetation. The spores were detected far from any agricultural zone in the dry desert town of Arad in Israel. This town deliberately maintains a minimal vegetation cover and has severe restrictions on plant species which are allowed to grow there, in order that it may serve as a spa for allergic disease patients (BARKAI-GOLAN et al. 1977).
The objectives of this study were to: (i) find out whether A. alternata spores are air-borne in the field, (ii) measure the distance these spores are transported by wind, (iii) evaluate the effect of wind and row direction on the rate of spread of Alternaria blight disease in the field, and (iv) correlate the relation between presence of infected plants and air-borne spores of A. alternata during the entire year.
Spores of Alternaria alternata (Fr.) Keissler were trapped and counted in this study. Cotton plants (Gossypium barbadense cv. Pima S-5 and G. birsutum cv. Acala SJ-2) were grown in commercial fields under the regular practice employed in cotton cultivation in Israel. No special arrangement of plants or treatment was given to the fields.
228 BASHAN, LEVANONY and OR Cotton plants designated for artificial inoculation (cv. Pima S-5) were grown from seeds in 5 1 pots in peat : vermiculite : volcanic dust (1 : 1 : 1, v/v/v), 3 plants/pot, in a net house (against bird damage). Plants were fertilized (100 ml/pot) with half-strength Hoagland's nutrient solution every week after germination. At the 3-5 true leaf stage, plants were inoculated with 12000 spores/ml of A. alternata strain S-1 (BASHAN et al. 1990) by brushing the leaves with an aqueous spore suspension containing a small amount of fine carborundum (BDH, 300 grid) (BASHAN et al. 1990). Each pot was then covered with a loosely sealed, large, pre-wetted polyethylene bag and incubated in the dark for 16 h at 25 ± 1 °C after which the plants were returned to the net house for an additional 5 days. Each plastic bag was removed daily for a few minutes to improve aeration.
Fungal cultures were grown and maintained on Czapeck (Dox) medium (C.M.I. 1968). Cultures originating from air-borne spores were treated as described later.
Disease severity index
Disease severity was recorded 8 days after inoculation using the following scale: 0 = no symptoms; 1 = 1-3 lesions/leaf; 2 = 4-10 lesions/leaf; 3 = 11-20 1esions/leaf; 4 = 21-301esions/ leaf and 5 = more than 30 lesions/leaf, indicating a heavy infection. The numbers of lesions on all inoculated leaves were counted separately. The mean number of lesions per leaf represents the disease severity per plant.
Monitoring air-borne spores of A. alternate in the field and greenhouse
A. alternata spores were trapped in the field throughout the year by passively exposing Petri dishes containing Czapeck medium supplemented with 250 mg/1 chloramphenicol (C.M.I. 1968) and 1 mg/1 of the surfactant Triton N-101 (alkylphenylpolyethyleneglycol to prevent overgrowth of the colonies on the medium (MADELIN 1987). Dishes were exposed for 2 h on a flat table mounted 1.5 m above ground level. This height is known to be optimal for trapping air-borne spores of various fungi (LYON et al. 1984 a). The table size was 30 x 40 cm and five Petri dishes per sampling time were mounted on each table using soft solid clay. Each dish was vertically oriented to the following wind directions: 90° (east), 180° (south), 270° (west) and 360° (north) and one dish was placed in an horizontal position facing upward. This setup allowed exposure of up to a total of 318 cm2 of medium surface to the air. The traps were located in the field as described later. Gravity traps of this kind are common in numerous field studies (BANERJEE et al. 1987, B ARKAI-GOLAN 1958, D URHAM 1946, HYRE 1950, GREGORY 1950, MADELIN 1987).
In the greenhouse (a commercial sealed polyethylene tunnel, 3 m in diameter, 22 m long with no environmental control such as temperature, light, humidity or CO2, used primarily for growth of winter vegetables in Israel), five highly infected cotton plants (before flowering stage, mean disease severity index of 3.8) were placed on a stand 0.5 m above ground level in front of the greenhouse ventilation system which produced a constant wind of 4 ± 1 m/sec along the longitudal dimension of the greenhouse. Two traps containing five Petri dishes per trap vertically facing the wind direction were exposed, in 1 m intervals for 2 h (a total of 200 Petri dishes/sampling).
After exposure of the medium, lids were returned, sealed with Parafilm, transferred to the laboratory, incubated at 25 ± 2 °C in the dark and spore formation was induced as previously described for A. macrospora (BASHAN 1984). Three to 5 days after sampling, each colony which developed was examined under a stereoscopic microscope for the presence of A. alternata according to the description of this species by SIMMONS (1967). Results are given as numbers of spores trapped/ 60-300 cm2 medium surface/2 h. Pathogenicity tests of the trapped air-borne spores were routinely carried out as previously described (BASHAN 1984).
Sites for monitoring Alternaria blight disease and A. alternata air-borne spores