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Citrus plantation fungi in upper Egypt
[ 289 ] Trans. Br, mycol. Soc. 57 (2), 28g-294 (1971) Printed in Great Britain
CITRUS PLANTATION FUNGI IN UPPER EGYPT By A. H. MOUBASHER, M. A. ELNAGHY, AND H. M. ABDEL-FATTAH
Botany Department, Assiut Unioersity, Egypt The fungi from soil under citrus trees, on the surfaces of sound citrus fruit and leaves and in the air of two plantations were sampled during the fruiting seasons of two consecutive years. The fungus flora of soil under five varieties of citrus was not specific, but basically similar to that in other Egyptian cultivated soils. The basic components were Aspergillus, comprising 43' 5 % of the total count of fungi as estimated by the dilution plate method, Fusarium (21'4%) and Penicillium (16'7%). In the air and on the surfaces of sound fruit and leaves, the fungal flora was considerably different from that in the soil. Cladosporium herbarum was most abundant in the air (76'7%) and on the surfaces of fruits (70.8%) and leaves (68'4%), but was poorly represented in soil (3'7%). Penicillium italicum was isolated once from the soil under grape-fruit, twice from the air and four and seven times from the rind and the leafsurfaces respectively. P. digitatum was not recovered from any substrate.
Attention has been paid to soil and air in citrus plantations and packing houses as sources of infestation of citrus fruit by pathogenic fungi. BenMeir-Glueck (1952) isolated more than thirty different species from the air of orange groves and packing sheds, and from the skins of fruits. These included Penicillium italicum and P. digitatum, which are the main incitants of citrus rot. Barkai-Golan (1961) studied the air-borne fungi in citrus fruit packing houses and reported that P. digitatum and P. italicum predominated, whereas Fusarium, Trichoderma, Colletotrichum and Diplodia were encountered only occasionally. Nairn (1967) isolated seventeen genera and about forty-four species offungi from soil under citrus trees. The dominant genera were Aspergillus, Penicillium, Alternaria, and Cladosporium. He identified five Penicillium species, including P. digitatum and P. italicum. Moubasher & Moustafa (1970) identified P. italicum in rare frequency among thirtythree species of Penicillium collected from thirty-two soil samples from different localities in Egypt. However, nothing is known about the prevalence of P. italicum and P. digitatum in citrus plantations. An extensive survey of these two species and other fungi in soil and air and on the surfaces of citrus fruits and leaves in two plantations was made in the Assiut area which is one of the main centres of citrus production in Egypt. MATERIALS AND METHODS
Determination ofsoil fungi in citrus plantations Soil samples were collected monthly from under trees of five types of citrus cultivated in two plantations during the fruiting seasons of 1967-8 and 1968-9. The Sahil-Salim plantation, 30 km from Assiut, is of about
Transactions British Mycological Society 60 acres and includes mandarin, sweet, and baladi oranges. In the first season sampling was between November, when the three citrus varieties are approaching maturity, and February, which is the last month of the fruiting season. In the second season, sampling was from July to February. The Assiut University Farm plantation is of about 2 acres and includes grapefruit and lemon. Sampling was between October and November in the first season and betweenJuly and November in the second. In sampling, the method of Johnson, Curl, Bond & Fribourg (1959) was modified. Samples were taken to a depth of 15 cm from the surface around superficial roots; 25 g (on an oven-dry basis) of the mixed composite sample were used for determining microbial numbers by the dilution plate method using a modified Czapek's agar in which glucose (10 gil) and K 2H P0 4 replaced sucrose and KH 2P0 4 respectively. Rose bengal, as a bacteriostatic agent, was added at a concentration of 1/15000 (Smith & Dawson, 1944). The six plates for each sample were incubated at 25°C and examined daily. The total count of fungi, as well as fungal genera and species, was determined at the end of the incubation period. Isolations of some fungal colonies were made on Czapek's agar containing 0'05 % yeast extract or on malt extract agar medium and the developing fungal species subsequently identified.
Determination offungal air-spore in citrus plantations Once every month during the fruiting seasons of 1967-8 and 1968-9, six Petri dishes of modified Czapek's agar containing 1/15000 rose bengal were exposed to the air for intervals of 20 s, between 10.00 hand 12.00 h at a height of 1·8 m in different sites of the two citrus plantations. In an experiment on air-borne fungi at Assiut, exposure was on the roof of the Botany Department which is about 15 m high for 10 min intervals. The plates were incubated at 25° and examined daily. Determination offungi on the surfaces of citrus fruits andleaves Samples of sound fruits of the five types of citrus were collected each month from several trees during the two fruiting seasons, placed in sterile plastic bags, then transferred immediately to the laboratory. Duplicate samples, each of five fruits in the case of oranges, lemons and mandarins and of two in the case of grapefruit, were washed for 10-15 min by shaking in a 21 jars containing 0'51 sterile water. The washings from duplicates were mixed together, and I ml of the mixture plated on modified Czapek's agar + 1/15000 rose bengal. Two samples, of thirteen leaves each, of each type of citrus were washed by shaking mechanically in a 2 ljars containing 0'51 sterile water. The washings from the duplicates were mixed and the fungal content estimated. RESULTS
The numerous data obtained are summarized in Table I. The counts of soil fungi, as estimated by the dilution plate method, reveal no specific fungus flora of the soil under citrus trees, and the twenty-eight genera and forty-one species recovered in the isolation plates have been previously
Citrus fungi. A. H. Moubasher and others
291
identified from Egyptian soils (Moubasher & Moustafa, 1970; Moubasher & Mazen, 1971). Moreover, no basic differences were observed between the fungal populations of the soils under the five types of citrus. In the air and on the surfaces offruits and leaves the fungi were similar to those in the soil but the percentage counts were considerably different. The commonest soil fungus was Aspergillus which constituted 43'5 % of the total count of fungi and was recovered in forty-nine isolations out of fifty. But percentages in the air and on the rind and the leaf surfaces were 5,8, 13-1, and 5'1 Table
I.
Number of isolations (N) andpercentage counts of the dominant fungi in soil, air, andon the surfaces offruit andleaves Source of samples A
Soil
Air
Leaf
Rind
,-------A-----. ,..-----'-----. ,---I------... ,..-----'-----.
N Aspergillus spp. A, niger van Tieg, .&, ochraceus Willi, Penicillium spp, P, notatum Westl, P, chrysogmum Thorn P, italicwn Wehmer Fusarium spp, Cladosporium herbarum Link ex Fr. Alternaria altemata (Nees) Simmons CurllUlaria spp, Helminthosporium satillUm Parnrn. King & Bakke Epicoccwn nigrum Link SttM:hybotrys atra Corda Stemphylium botryosum Wallr,
N
N
N
(50)·
%
(20)·
%
(50)·
%
(50)·
%
49 45 31
20 J7 8 3 3 2 14
5.8 2'5 1'4 4'7 0'[ 0'[ 3'2 4'5
49
31 17 1 49
43'5 39'0 8'1 16'7 4'1 1'5 0'02 21"4
26 4:2 :26 9 4 46
J3'1 8,8 1'4 3,8 0'4 0'04 0'04 3'0
47 47 22 38 22 14 7
48
5'1 3'9 0'2 1'4 0'3 0'1 0'1 :2'[
16
3'7
18
76'7
49
7° ·8
45
68'4
3 14
0'04 1'2
15 18
2'3 2,8
48
42
5,6 1"6
49 37
6,8 1'1
2 8 5 3
0'07 0'2 0'1 0'04
12 9 1 1
2'3 J'2 0'04 0'04
25 29 J4 5
0,6 0'7 0'2 0'1
20 29
0'4 0'7 0'1 0'[
44-
11
48
11
8
• Total possible isolations
respectively. Twelve Aspergillus species were collected from the four sources, and A_ niger was consistently the most dominant. The genus predominated (40-80 % of the monthly counts of fungi) in soil from August to October which is the late summer to autumn period characterized by relatively moderate temperatures. Aspergillus predominates over Penicillium in Egyptian and tropical soils, but the reverse occurs in temperate zones, In the air, Penicillium notably surpassed Aspergillus respectively constituting 6'1 and 5-4 % of the total fungi at Manhattan, Kansas (Kramer, Pady, Rogerson & Ouye, 1959), 1-8 and 1'7 % at Samaru, N_ Nigeria (Dransfield, 1966), 17'1 and 10-5 % in Hong Kong (Turner, 1966),8'1 and 0'7 % at Nottingham (Pawsey & Heath, 1964), and 2'5-2'8 % and 1'1-1 '4 % at Cambridge (Hudson, 1969). In the two citrus plantations and at Assiut (unpublished work), Aspergillus regularly exceeded Penicillium in percentage count and in number of cases of isolation. Fusarium was the second commonest genus in the soil (21'4 %), with percentages in the air, on the rind, and on the leaf surface of 45, 3'0 and 2'1 respectively. It was represented by four species, namely F, moniliforme
292
Transactions British Mycological Society
Sheld., F. oxysporum Schlecht., F. semitectum Berk. & Rav., and F. solani (Mart.) App. & Wr. F. moniliforme was the commonest in soil and air. Fusarium has been trapped also in low counts of 0·8 % from the air at Manhattan by Kramer et al. (1959) and 1'1 % at Hong Kong by Turner (1966) but it contributed 14'4 % of the total fungal spores trapped in a rice field in India (Sreeramulu & Seshavataram, 1962) and 8'2 % of the fungal colonies recovered in the exposure plates of Dransfield (1966) at Samaru. In the soil of the two citrus plantations, Fusarium predominated (30-60 % of the monthly counts of fungi) during summer months. Penicillium constituted 16'7 % of the total count in soil under citrus trees, 4'7% in the air, 3.8% on the rind surface, and 1'4% on the leaf surface. The genus showed its highest preponderence in soil during November to December which is a winter period. Thirteen species were collected from the four sources, of which P. notatum, P. chrysogenum and P. corylophilum were dominant. P. italicum, one of the two main incitants of Penicillium-rot, was isolated once from the soil under grapefruit. From the air it was trapped once in December 1968 and once in February 1969. It was also recovered four times from the rind of sound citrus fruits, twice from baladi oranges and once from sweet oranges and mandarin. From leaves it was isolated three times from baladi oranges, twice from mandarin and once from sweet oranges and lemon. Many fruits of the two orange varieties and mandarin were severely infected with P. italicum while attached to the trees. Cladosporium herbarum was the commonest fungus in the air of the two citrus plantations. In contributed 76'7 % of the total fungal isolations although it was missed in two exposures out of twenty. It was more abundant than any other fungus during most of the months of the experiment. Its highest incidence was observed during November to December. Sreeramula & Seshavataram (1962) observed that Cladosporium was most abundant during November to December in the air over rice fields in India. C. herbarum was also most common at Assiut where it comprised 49'9 % of the total colonies offungi trapped from the air. Cladosporium has also been reported as the commonest air-borne organism in many temperate and tropical localities; 47'7 % at Montreal (Pady & Kapica, 1955), 44'5 % at Manhattan (Kramer et al. 1959), 51.6 % at Cardiff (Hyde & Williams, 1953), 69.6 % at nine sites in Britain (Richards, 1956), 73 % at Nottingham (Pawsey & Heath, 1964), 78'8-79'7 % at Cambridge (Hudson, 1969), 36.8 % at Samaru, N. Nigeria (Dransfield, 1966), and 65'7 % at Hong Kong (Turner, 1966). This organism was also extremely abundant on the rind and leaf surfaces (70'8 % and 68'4 % respectively), but was infrequent in soil (3'7 %). Three Cladosporium species, of which C. herbarum was the commonest, have been found in Egyptian soils (Moubasher & Mazen, 1971). DISCUSSION
Melchers (1931) and Jones (1935) reported P. italicum and P. digitatum Sacco as causal agents of citrus-rot in Egypt. P. digitatum was not recovered in the present survey. Either it was actually absent or it was so rare in the soil, air and on the surfaces ofleaves and fruits that it was missed, or it was
Citrus fungi. A. H. Moubasher and others
293
overgrown in the isolation plates by other fast-growing fungi. It is well known that P. digitatum grows weakly on Czapek's agar. Elnaghy, Moubasher & Abdel-Fattah (1971) reported that P. italicum was the sole incitant of Penicillium-rot in the Assiut area. P. italicum was only isolated once in the soil under grapefruit, where it accounted for 300 units per g dry soil. This may represent a source of fruit infestation. It was twice trapped from the air and became a basic component of the air fungal spora in December 1969 when there were 17'7 colonies per plate, it consequently gained the highest total count among all Penicillium species. This point is of considerable interest as it shows that citrus fruits are subject to infestation by the spores of P. italicum suspended in the air and that these will be the inoculum for rot if conditions are favourable. There is substantial evidence that citrus fruits are infested with P. italicum in the field because it was recovered four times from the surface of rind of sound fruits and, in addition, there were many Penicillium-infected fruits rotting while attached to the trees. The dominance of C. herbarum in the air-spora in citrus plantations and on the surfaces of sound fruits and leaves and its poor representation in the soil under citrus trees may give further support for the view of Gregory (1961) that fungal air spora are essentially a contribution from plant and vegetable debris rather than from the soil itself. The authors are very grateful to all members of the Commonwealth Mycological Institute, Kew, England, for kindly checking the identification of some species isolated in this investigation.
REFERENCES
BARKAI-GOLAN, R. (1961). Air-borne fungi in packinghouses for citrus fruits. Bulletinof the Research Council of Israel, D 10, 135-141. BEN-MEIR-GLUECK, Y. (1952). Fungi isolated from the air of groves and packing sheds and from the skin of Shamouti oranges. Ktaoim 2-3, 135-I40. DRANSFIELD, M. (1966). The fungal air-spora at Samaru, Northern Nigeria. Transactions of the British Mycological Society 49, 121-132. ELNAGHY, M. A., MOUBASHER, A. H. & ABDEL-FATTAH, H. M. (1971). Pathogenicity of Penicillium italicum Wehmer and other fungi of Egyptian citrus fruits. Assiut University Bulletin of Science and Technology. FREY, D. & DURm, E. B. (1960). The incidence of air-borne fungi in Sydney. Mycopathologia et Mycologia Applicata 13, 93-99. GREGORY, P. H. (1961). The microbiology of theatmosphere. London: Leonard Hill. HUDSON, H. J. (1969). Aspergilli in the air spora at Cambridge. Transactions oftheBritish Mycological Society 52, 153-159. HYDE, H. A. & WILLIAMS, D. A. (1953). The incidence of Cladosporium herbarum in the outdoor air at Cardiff, 1949-50. Transactions of the British Mycological Society 36, 260-66. JOHNSON, L., CURL, E., BOND,J. & FRIBOURG, H. (1959). Methodsfor studying soilmicrojloraplant disease relationships. Minneapolis: Burgess Publishing Co. JONES, G. H. (1935). Egyptian plant diseases: a summary of research and control methods. Bulletin. Ministry of Agriculture, Egypt. Technical and Scientific Service, no. 146. KRAMER, C. L., PADY, S. M., ROGERSON, C. T. & OUYE, L. G. (1959). Kansas aeromycology. II: Materials, method and general results. Transactions of the Kansas Academy ofScience 62, 184-199. MELCHERS, L. E. (1931). A check list of plant diseases and fungi occurring in Egypt. Transactions of the Kansas Academy of Science 34, 41-106.
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Transactions British Mycological Society
MOUBASHER, A. H. & MAZEN, M. B. (1971). Dematiaceous Hyphomycetes in Egyptian soils. Transactions of the British Mycological Society. MOUBASHER, A. H. & MOUSTAFA, A. F. (1970). A survey of Egyptian soil fungi with special reference to Aspergillus, Penicillium, and Pencillium-related genera. Transactions of the British Mycological Society 54, 33-44· NAIM, M. S. (1967)' Contribution to the knowledge of soil fungi in Libya. II. Fungus flora under citrus trees in Libya. Mycopathologia et Mycologia Applicata 31, 300-304PADY, S. M. & KAPICA, L. (1956). Fungi in air masses over Montreal during 1950-1951. Canadian Journal of Botany 34, 1-15. PAWSEY, R. G. & HEATH, L. A. F. (1964). An investigation of the spore population of the air at Nottingham. I. The results of Petri dish trapping over one year. Transactions of the British Mycological Society 47, 351-355. RICHARDS, M. (1956). A census of mould spores in the air over Britain in 1952. Transactions of the British Mycological Society 39,431-441. SMITH, N. R. & DAWSON, V. T. (1944). The bacteriostatic action of rose bengal in media used for the plate counts of soil fungi. Soil Science 58, 467-471. SREERAMULU, T. & SESHAVATARAM, V. (1962). Spore content of air over paddy fields. I. Changes in a field near Pentapadu from 21 September to 31 December 1957. Indian Phytopathology 15, 61-74. TuRNER, P. D. (1966). The fungal air spora of Hong Kong as determined by the agar plate method. Transactions of the British Mycological Society 49, 255-267.
(Accepted for publication 28 April 197I )
CITRUS PLANTATION FUNGI IN UPPER EGYPT By A. H. MOUBASHER, M. A. ELNAGHY, AND H. M. ABDEL-FATTAH
Botany Department, Assiut Unioersity, Egypt The fungi from soil under citrus trees, on the surfaces of sound citrus fruit and leaves and in the air of two plantations were sampled during the fruiting seasons of two consecutive years. The fungus flora of soil under five varieties of citrus was not specific, but basically similar to that in other Egyptian cultivated soils. The basic components were Aspergillus, comprising 43' 5 % of the total count of fungi as estimated by the dilution plate method, Fusarium (21'4%) and Penicillium (16'7%). In the air and on the surfaces of sound fruit and leaves, the fungal flora was considerably different from that in the soil. Cladosporium herbarum was most abundant in the air (76'7%) and on the surfaces of fruits (70.8%) and leaves (68'4%), but was poorly represented in soil (3'7%). Penicillium italicum was isolated once from the soil under grape-fruit, twice from the air and four and seven times from the rind and the leafsurfaces respectively. P. digitatum was not recovered from any substrate.
Attention has been paid to soil and air in citrus plantations and packing houses as sources of infestation of citrus fruit by pathogenic fungi. BenMeir-Glueck (1952) isolated more than thirty different species from the air of orange groves and packing sheds, and from the skins of fruits. These included Penicillium italicum and P. digitatum, which are the main incitants of citrus rot. Barkai-Golan (1961) studied the air-borne fungi in citrus fruit packing houses and reported that P. digitatum and P. italicum predominated, whereas Fusarium, Trichoderma, Colletotrichum and Diplodia were encountered only occasionally. Nairn (1967) isolated seventeen genera and about forty-four species offungi from soil under citrus trees. The dominant genera were Aspergillus, Penicillium, Alternaria, and Cladosporium. He identified five Penicillium species, including P. digitatum and P. italicum. Moubasher & Moustafa (1970) identified P. italicum in rare frequency among thirtythree species of Penicillium collected from thirty-two soil samples from different localities in Egypt. However, nothing is known about the prevalence of P. italicum and P. digitatum in citrus plantations. An extensive survey of these two species and other fungi in soil and air and on the surfaces of citrus fruits and leaves in two plantations was made in the Assiut area which is one of the main centres of citrus production in Egypt. MATERIALS AND METHODS
Determination ofsoil fungi in citrus plantations Soil samples were collected monthly from under trees of five types of citrus cultivated in two plantations during the fruiting seasons of 1967-8 and 1968-9. The Sahil-Salim plantation, 30 km from Assiut, is of about
Transactions British Mycological Society 60 acres and includes mandarin, sweet, and baladi oranges. In the first season sampling was between November, when the three citrus varieties are approaching maturity, and February, which is the last month of the fruiting season. In the second season, sampling was from July to February. The Assiut University Farm plantation is of about 2 acres and includes grapefruit and lemon. Sampling was between October and November in the first season and betweenJuly and November in the second. In sampling, the method of Johnson, Curl, Bond & Fribourg (1959) was modified. Samples were taken to a depth of 15 cm from the surface around superficial roots; 25 g (on an oven-dry basis) of the mixed composite sample were used for determining microbial numbers by the dilution plate method using a modified Czapek's agar in which glucose (10 gil) and K 2H P0 4 replaced sucrose and KH 2P0 4 respectively. Rose bengal, as a bacteriostatic agent, was added at a concentration of 1/15000 (Smith & Dawson, 1944). The six plates for each sample were incubated at 25°C and examined daily. The total count of fungi, as well as fungal genera and species, was determined at the end of the incubation period. Isolations of some fungal colonies were made on Czapek's agar containing 0'05 % yeast extract or on malt extract agar medium and the developing fungal species subsequently identified.
Determination offungal air-spore in citrus plantations Once every month during the fruiting seasons of 1967-8 and 1968-9, six Petri dishes of modified Czapek's agar containing 1/15000 rose bengal were exposed to the air for intervals of 20 s, between 10.00 hand 12.00 h at a height of 1·8 m in different sites of the two citrus plantations. In an experiment on air-borne fungi at Assiut, exposure was on the roof of the Botany Department which is about 15 m high for 10 min intervals. The plates were incubated at 25° and examined daily. Determination offungi on the surfaces of citrus fruits andleaves Samples of sound fruits of the five types of citrus were collected each month from several trees during the two fruiting seasons, placed in sterile plastic bags, then transferred immediately to the laboratory. Duplicate samples, each of five fruits in the case of oranges, lemons and mandarins and of two in the case of grapefruit, were washed for 10-15 min by shaking in a 21 jars containing 0'51 sterile water. The washings from duplicates were mixed together, and I ml of the mixture plated on modified Czapek's agar + 1/15000 rose bengal. Two samples, of thirteen leaves each, of each type of citrus were washed by shaking mechanically in a 2 ljars containing 0'51 sterile water. The washings from the duplicates were mixed and the fungal content estimated. RESULTS
The numerous data obtained are summarized in Table I. The counts of soil fungi, as estimated by the dilution plate method, reveal no specific fungus flora of the soil under citrus trees, and the twenty-eight genera and forty-one species recovered in the isolation plates have been previously
Citrus fungi. A. H. Moubasher and others
291
identified from Egyptian soils (Moubasher & Moustafa, 1970; Moubasher & Mazen, 1971). Moreover, no basic differences were observed between the fungal populations of the soils under the five types of citrus. In the air and on the surfaces offruits and leaves the fungi were similar to those in the soil but the percentage counts were considerably different. The commonest soil fungus was Aspergillus which constituted 43'5 % of the total count of fungi and was recovered in forty-nine isolations out of fifty. But percentages in the air and on the rind and the leaf surfaces were 5,8, 13-1, and 5'1 Table
I.
Number of isolations (N) andpercentage counts of the dominant fungi in soil, air, andon the surfaces offruit andleaves Source of samples A
Soil
Air
Leaf
Rind
,-------A-----. ,..-----'-----. ,---I------... ,..-----'-----.
N Aspergillus spp. A, niger van Tieg, .&, ochraceus Willi, Penicillium spp, P, notatum Westl, P, chrysogmum Thorn P, italicwn Wehmer Fusarium spp, Cladosporium herbarum Link ex Fr. Alternaria altemata (Nees) Simmons CurllUlaria spp, Helminthosporium satillUm Parnrn. King & Bakke Epicoccwn nigrum Link SttM:hybotrys atra Corda Stemphylium botryosum Wallr,
N
N
N
(50)·
%
(20)·
%
(50)·
%
(50)·
%
49 45 31
20 J7 8 3 3 2 14
5.8 2'5 1'4 4'7 0'[ 0'[ 3'2 4'5
49
31 17 1 49
43'5 39'0 8'1 16'7 4'1 1'5 0'02 21"4
26 4:2 :26 9 4 46
J3'1 8,8 1'4 3,8 0'4 0'04 0'04 3'0
47 47 22 38 22 14 7
48
5'1 3'9 0'2 1'4 0'3 0'1 0'1 :2'[
16
3'7
18
76'7
49
7° ·8
45
68'4
3 14
0'04 1'2
15 18
2'3 2,8
48
42
5,6 1"6
49 37
6,8 1'1
2 8 5 3
0'07 0'2 0'1 0'04
12 9 1 1
2'3 J'2 0'04 0'04
25 29 J4 5
0,6 0'7 0'2 0'1
20 29
0'4 0'7 0'1 0'[
44-
11
48
11
8
• Total possible isolations
respectively. Twelve Aspergillus species were collected from the four sources, and A_ niger was consistently the most dominant. The genus predominated (40-80 % of the monthly counts of fungi) in soil from August to October which is the late summer to autumn period characterized by relatively moderate temperatures. Aspergillus predominates over Penicillium in Egyptian and tropical soils, but the reverse occurs in temperate zones, In the air, Penicillium notably surpassed Aspergillus respectively constituting 6'1 and 5-4 % of the total fungi at Manhattan, Kansas (Kramer, Pady, Rogerson & Ouye, 1959), 1-8 and 1'7 % at Samaru, N_ Nigeria (Dransfield, 1966), 17'1 and 10-5 % in Hong Kong (Turner, 1966),8'1 and 0'7 % at Nottingham (Pawsey & Heath, 1964), and 2'5-2'8 % and 1'1-1 '4 % at Cambridge (Hudson, 1969). In the two citrus plantations and at Assiut (unpublished work), Aspergillus regularly exceeded Penicillium in percentage count and in number of cases of isolation. Fusarium was the second commonest genus in the soil (21'4 %), with percentages in the air, on the rind, and on the leaf surface of 45, 3'0 and 2'1 respectively. It was represented by four species, namely F, moniliforme
292
Transactions British Mycological Society
Sheld., F. oxysporum Schlecht., F. semitectum Berk. & Rav., and F. solani (Mart.) App. & Wr. F. moniliforme was the commonest in soil and air. Fusarium has been trapped also in low counts of 0·8 % from the air at Manhattan by Kramer et al. (1959) and 1'1 % at Hong Kong by Turner (1966) but it contributed 14'4 % of the total fungal spores trapped in a rice field in India (Sreeramulu & Seshavataram, 1962) and 8'2 % of the fungal colonies recovered in the exposure plates of Dransfield (1966) at Samaru. In the soil of the two citrus plantations, Fusarium predominated (30-60 % of the monthly counts of fungi) during summer months. Penicillium constituted 16'7 % of the total count in soil under citrus trees, 4'7% in the air, 3.8% on the rind surface, and 1'4% on the leaf surface. The genus showed its highest preponderence in soil during November to December which is a winter period. Thirteen species were collected from the four sources, of which P. notatum, P. chrysogenum and P. corylophilum were dominant. P. italicum, one of the two main incitants of Penicillium-rot, was isolated once from the soil under grapefruit. From the air it was trapped once in December 1968 and once in February 1969. It was also recovered four times from the rind of sound citrus fruits, twice from baladi oranges and once from sweet oranges and mandarin. From leaves it was isolated three times from baladi oranges, twice from mandarin and once from sweet oranges and lemon. Many fruits of the two orange varieties and mandarin were severely infected with P. italicum while attached to the trees. Cladosporium herbarum was the commonest fungus in the air of the two citrus plantations. In contributed 76'7 % of the total fungal isolations although it was missed in two exposures out of twenty. It was more abundant than any other fungus during most of the months of the experiment. Its highest incidence was observed during November to December. Sreeramula & Seshavataram (1962) observed that Cladosporium was most abundant during November to December in the air over rice fields in India. C. herbarum was also most common at Assiut where it comprised 49'9 % of the total colonies offungi trapped from the air. Cladosporium has also been reported as the commonest air-borne organism in many temperate and tropical localities; 47'7 % at Montreal (Pady & Kapica, 1955), 44'5 % at Manhattan (Kramer et al. 1959), 51.6 % at Cardiff (Hyde & Williams, 1953), 69.6 % at nine sites in Britain (Richards, 1956), 73 % at Nottingham (Pawsey & Heath, 1964), 78'8-79'7 % at Cambridge (Hudson, 1969), 36.8 % at Samaru, N. Nigeria (Dransfield, 1966), and 65'7 % at Hong Kong (Turner, 1966). This organism was also extremely abundant on the rind and leaf surfaces (70'8 % and 68'4 % respectively), but was infrequent in soil (3'7 %). Three Cladosporium species, of which C. herbarum was the commonest, have been found in Egyptian soils (Moubasher & Mazen, 1971). DISCUSSION
Melchers (1931) and Jones (1935) reported P. italicum and P. digitatum Sacco as causal agents of citrus-rot in Egypt. P. digitatum was not recovered in the present survey. Either it was actually absent or it was so rare in the soil, air and on the surfaces ofleaves and fruits that it was missed, or it was
Citrus fungi. A. H. Moubasher and others
293
overgrown in the isolation plates by other fast-growing fungi. It is well known that P. digitatum grows weakly on Czapek's agar. Elnaghy, Moubasher & Abdel-Fattah (1971) reported that P. italicum was the sole incitant of Penicillium-rot in the Assiut area. P. italicum was only isolated once in the soil under grapefruit, where it accounted for 300 units per g dry soil. This may represent a source of fruit infestation. It was twice trapped from the air and became a basic component of the air fungal spora in December 1969 when there were 17'7 colonies per plate, it consequently gained the highest total count among all Penicillium species. This point is of considerable interest as it shows that citrus fruits are subject to infestation by the spores of P. italicum suspended in the air and that these will be the inoculum for rot if conditions are favourable. There is substantial evidence that citrus fruits are infested with P. italicum in the field because it was recovered four times from the surface of rind of sound fruits and, in addition, there were many Penicillium-infected fruits rotting while attached to the trees. The dominance of C. herbarum in the air-spora in citrus plantations and on the surfaces of sound fruits and leaves and its poor representation in the soil under citrus trees may give further support for the view of Gregory (1961) that fungal air spora are essentially a contribution from plant and vegetable debris rather than from the soil itself. The authors are very grateful to all members of the Commonwealth Mycological Institute, Kew, England, for kindly checking the identification of some species isolated in this investigation.
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(Accepted for publication 28 April 197I )