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Effect of water droplets on the development of Sphaerotheca pannosa on rose leaves
[ 313 ] Trans. Br. mycol. Soc. 64 (2), 313-319 (1975) Printed in Great Britain
EFFECT OFWATER DROPLETS ONTHEDEVELOPMENT OF SPHAEROTHECA PANNOSA ON ROSE LEAVES By R. G. PERERA*
AND
B. E. J. WHEELER
Imperial College Field Station, Silwood Park, Sunninghill, Berks. (With
1
Text-figure)
The development of Sphaerotheca pannosa (Wallr.) Lev. on detached rose leaves was retarded by a water spray especially when this was applied immediately after the leaves were inoculated with conidia. Similar numbers of conidia germinated on wetted and non-wetted leaves but fewer germinated conidia grew to sporulating colonies on mist-sprayed leaves and more remained at the single germ-tube stage suggesting that water affected particularly the initial penetration of the host.
It is well established that water inhibits germination of powdery mildew conidia (e.g. Corner, I93S; Yarwood, 1936; Longree, 1939; Brodie & Neufeld, 1942; Brodie, 1945; Delp, 1954; Schnathorst, 1960; Manners & Hossain, 1963; Morrison, 1964; Zaracovitis, 1964). There are reports also which indicate that when water is applied to plants mildew growth is impaired. Yarwood (1936), for example, found that mildew on peas was destroyed by spraying infected leaves and later reported that water sprays controlled powdery mildews on rose, bean, cucumber and barley (Yarwood, 1939). Though spraying with water does not appear to be commercially satisfactory for the control of rose powdery mildew (McClellan, 1942), information on the ways in which water limits mildew is important in relation to epidemiological studies of the disease (Rogers, 1959). The effect of water on the development of Sphaerotheca pannosa on rose leaves, particularly in its early stages, was investigated in the experiments described below. MATERIALS AND METHODS
The miniature rose cultivar, Perle d'Alcanada was grown in John Innes no. 1 compost, singly in pots (IS'5 cm diam) in the open. Every 10 days, five to eight plants were severely pruned and transferred to a glasshouse at 17-19 °C with a 16 h photoperiod to encourage the production of new shoots. Leaves which had just expanded were used in the experiments and they were inoculated with conidia of S. pannosa as described by Price (1970). The conidia were obtained from cultures on the adaxial surface of young rose leaves, cv. Little Buckeroo, which were floated on distilled water in polystyrene boxes (5'5 x 3'5 x 2'0 cm) and incubated in a cabinet at 17 ± 1 °C with a 16 h photoperiod.
*
Present address: Botany Department, University of Ceylon, Colombo 3.
314
Transactions British Mycological Society
Inoculated leaves were placed in polystyrene boxes (17,8 x 10'2 x 3,8 em) on wet tissue and incubated at 20 ± I "C. Where required inoculated leaves were covered with many minute water droplets by spraying them with a fine mist of distilled water from a Shandon spray gun held about 60-90 em away. At the end of the required wet period leaves were dried under a fan placed 120-150 em away for c. 30 min and were then reincubated in the moist chamber. Growth of S. pannosa on the leaves was assessed as a growth index from celloidin strips stained in cotton-blue-lactophenol: h . d growt
III
sum of grades 100 ex = no. conidia assessed x highest rating (=4)'
There were five grades based on the stages in the development of the fungus over 72 h as indicated by Price (1970): 0, ungerminated conidium; I, single germ-tube and appressorium; 2, appearance of second germtube; 3, appearance of third germ-tube, extension of first and second germ-tubes; 4, branching of germ tubes, production of conidiophore initials and sporulation. Assessments were based on three leaf samples from each treatment. Normally three to four celloidin strips were made of each sample and a total of one hundred conidia were assessed at random on these strips. Treatments were completely randomized within experiments and differences between treatment means were evaluated using Duncan's new multiple range test. EXPERIMENTAL
Expt I. Leaves were sprayed with water immediately after inoculation with conidia of S, pannosa and were kept wet for 2, 4, 6 or 8 h. Growth of the fungus on these leaves was compared with that on leaves not wetted. In this experiment growth was assessed both as an index and as total mycelial growth per 100 conidia. For the latter a tracing was made with a camera lucida of the germ-tube length of each conidium which was graded. The total length was then measured with an opisometer and related to a scale drawn at the same magnification from a stage micrometer. Both methods demonstrated that mildew development was adversely affected by wetting, the extent of this being related to the duration of the wet period (Fig. I). In the later stages, however, differences in growth were better expressed in terms of total mycelial length. This reflected more closely the marked visual differences in mildew growth on leaves from these treatments after 7 days. The relationship between the two measures of growth for the various samples of this experiment is given by the equation: y = 0'I4I4x- 1.8846 (correlation coefficient, 0'97) where x = growth indexand j = loge mycelial length. Expt 2. Leaves were subjected to wet periods of 4, 6 or 8 h either immediately following inoculation with conidia or 6 h later. The results (Table I) confirmed that wet periods of 6 and 8 h following inoculation markedly reduced mildew growth. Wetting leaves 6 h after inoculation with conidia appeared to have a less drastic effect.
Sphaerotheca pannosa. R. G. Perera and B. E.]. Wheeler
315
80
""' 8
-S
.c ~
8tlIl 60
ii
~
~
40
6
24
48
72
Time of incubation (h) Fig.
I.
Growth of Sphaerotheca pannosa on leaves kept wet for varying periods after inoculation .
Expt 3. Leaves were subjected to a wet period of 8 h either immediately after inoculation or 6 h later. Growth at 24, 48 and 72 h was significantly reduced by the former treatment (Table 2a). Wetting leaves 6 h after inoculation appeared only to lead to a temporary check (at 48 h) and by 7'2 h there was no significant difference in growth between this treatment and the control. An analysis of the growth stages reached by conidia (T able '2 b) showed that even on non-wetted leaves some 25 % of the
Transactions British Mycological Society Table I. Growth after 48 h of Sphaerotheca pannosa on leaves kept wet for varying periods immediately after inoculation (A) or 6 h later (B) Growth index
Table
Duration of wet period (h)
Control
A
B
4 6 8
44'4 48'1 56'5
45'0 33'5 21'5
45'3 44,6 30'6
Growth of Sphaerotheca pannosa on leaves given an 8 h wet period immediately after inoculation (A) or 6 h later (B)
2.
(a) Overall growth Incubation (h)
Mean growth index A
\
A
Treatment
6,6
B
Control
7'2 25'5 37'3 47'8
7'7 23'8 47'6 5°'7
Means within one assessment not underscored by the same (or any) line differ significantly (P < 0'05), (b) Growth stages reached Mean no, conidia in each grade Incubation (h) 24
A
Grade ° 2 ° 2 3 4 ° 2 3 4
Treatment
.,
,
A
B
Control
25'3 73'0 1'7
22'0 58'0 20'0
24'0 56'7 19'3
26'3 5°'7 2'7 II'3 g'o 27'0 56'0 1'7 3'0 12'3
22'3 47'0 5"7 g'o 16'0 24'7 29'3 5'7 10'7 29'7
21'3 30'0 8'3 17'7 22'7 25'0 28'7 1'7 7"7 37'0
Means within one grade not underscored by the same line differ significantly (P < 0'05)'
conidia never germinated and about the same number formed one germtube only after 72 h. Only 37 % of the total conidial population grew to a sporulating colony. Wetting leaves immediately after inoculation reduced the number of sporulating colonies and correspondingly, increased the number of conidia which developed only one germ-tube. The check in overall growth at 48 h on leaves wetted 6 h after inoculation stemmed from similar effects but some conidia at the single germ-tube stage then developed further.
Sphaerotheca pannosa. R. G. Perera and B. E.J. Wheeler
317
Table 3. Growth of Sphaerotheca pannosa on leaoes subjected to
different wetting treatments Growth index" after Treatment
\
2h
24 h
36 h
48 h
72 h
96 h
19'0 18'5 20'6 15'0 20'9
25"6 21'7 29'5 22'3 23'0
49'0 4 1'7 40'0 22'8 33'7
53'4 43'4 44,8 28'9 32'9
58 '0 5 1'5 5°'0 36'8 46 '0
56 '5 55'0 58'7
!
No wetting 8 h wet period, 12 h after inoculation 8 h wet period, 24 h after inoculation 12 h wet period immediately after inoculation 12 h wet period , 12 h after inoculation
• Mean of three replicates,
Table 4. Growth of Sphaerotheca pannosa on leaoes with water
droplets for 8 h after inoculation Growth index after 72 h , In areas with water droplets In non-wetted for 8h areas I
Leaf no, I
2
3° '0 21'4 34'2
3 4
41' 1
5 Mean
45'8 34'5
64'0 79'4 78 '9 86,8 67'8 75'4
Expt 4. Leaves were given an 8 h wet period at 12 and 24 h after inoculation. In these treatments growth was slightly less than the controls up to 72 h (T able 3) but by 96 h even these slight differences were no longer apparent. Expt 5. Leaves were kept wet for 12 h immediately after inoculation or 12 h later. Mildew growth was reduced by both treatments (Table 3) but particularly so where wetting immediately followed inoculation. This experiment was set up at the same time as Expt 4 and the control treatment was common to both. Expt 6. Six 2 mm diam rings were marked with Indian ink on the terminalleaflets of each of six leaves and when the ink dried the leaves were inoculated with conidia. Drops of distilled water were placed within three of the marked rings on each leaflet using an Agla syringe, the remaining marked areas were left dry. The leaflets were incubated for 8 h in a polystyrene box lined with wet tissue at 20 DC and then were air dried and immediately re-incubated. Celloidin peels of the marked areas were made 64 h later. The results (Table 4) indicated that mildew growth in the wetted areas was only about half that of the controls. DISCUSSION
The results indicate that the development of powdery mildew on detached rose leaves was retarded by spraying these with water. The effect was most marked and persistent when leaves were wetted imme-
318
Transactions British Mycological Society
diately after inoculation with conidia and the degree to which growth was retarded was directly related to the length of the wet period. Leaf wetness did not appear to inhibit the production of the first germtube by the conidium. On the contrary, germination on mist-sprayed leaves was only marginally less than on dry leaves during the first 6 h. It did, however, reduce the number of germinated conidia that grew to a sporulating colony and increased the number that remained at the r-germ-tube stage. Evidently the most susceptible phase occurs some 6-8 h after inoculation when processes leading to penetration of the host tissue are initiated. Possibly conidia are held above the leaf on the surfaces of the water droplets and the fungus is thus prevented from establishing contact with the host. Certainly in experiments where leaves were sprayed 6 or 12 h after inoculation with conidia, the inhibitory effect on growth was less marked. This suggests that fungal development following initial penetration of the host is much less sensitive to water, though here results must be interpreted with some caution because above values of 35 small differences in growth indices correspond to substantial differences in mycelial extension. Periods ofleafwetness following the initial penetration of the leaf could well have a considerable effect in delaying sporulation of the colony. Even water sprays applied immediately after inoculation with conidia did not completely inhibit mildew development. In this instance it is likely that colonies developed mainly from conidia which were between water droplets. In the field a similar situation could arise during periods of rain for the very young rose leaves which are most susceptible to mildew are those most difficult to wet and on these water stays in discrete droplets. REFERENCES
BRODIE, H.]. (1945). Further observations of the mechanism of germination of the conidia of various species of powdery mildew at low humidity. Canadian Journal of Research C 23, 198-2 I I. BRODIE. H.J. & NEUFELD. C. C. (1942). The development and structure of the conidia of Erysiphe polygoni DC. and their germination at low humidity. Canadian Journal of Research C 20, 41-62. CORNER, E.]. H. (1935). Observations on resistance to powdery mildews. New Phytologist 34, 180-200. DELP, C. L. (1954). Effect of temperature and humidity on the grape powdery mildew fungus. Phytopathology 44,615-626. LONGREE, K. (1939). The effect of temperature and relative humidity on the powdery mildew of roses. Cornell Agricultural Experiment Station Memoir no. 223,43 pp. MANNERS, J. G. & HOSSAIN, S. M. M. (1963). Effect of temperature and humidity on conidial germination in Erysiphe graminis. Transactions of the British Mycological Society 46, 225-234. MCCLELLAN, W. D. (1942). Control of powdery mildew of roses in the greenhouse. Carnell University Agricultural Experiment Station Bulletin no. 785, 39 pp. MORRISON, R. M. (1964). Germination of conidia of Erysiphe cichoracearum. Mycologia 56, 232-236. PRICE, T. V. (1970). Epidemiology and control of powdery mildew (Sphaerotheca pannasa) on rose. Annalsof AppliedBiology 65,231-248. ROGERS, M. N. (1959). Some effects of moisture and host plant susceptibility on the development of powdery mildew of roses caused by Sphaerotheca pannosa var, rosae. Carnell Agricultural Experiment Station Memoir no. 363, 38 pp.
Sphaerotheca pannosa. R. G. Perera and B. E.J. Wheeler 319
w. C. (1960). Effect of temperature and moisture stress on the lettuce powdery mildew fungus. Phytopathology 50, 304-308. YARWOOD, C. E. (1936). The tolerance of Erysiphe polygoni and certain other powdery mildews to low humidity. Phytopathology 26, 845-859. YARWOOD, C. E. (1939). Control of powdery mildews with a water spray. Phytopathology 29, 288-29° . ZARACOVITIS, C. (1964). Factors in testing fungicides against powdery mildews. The germination of conidia in vitro. Annals ofthe Phytopathologual Institute, Benaki N.5. 6, 73- 106. SCHNATHORST,
(Accepted for publication
21
I I
July 1974)
MYC
64
EFFECT OFWATER DROPLETS ONTHEDEVELOPMENT OF SPHAEROTHECA PANNOSA ON ROSE LEAVES By R. G. PERERA*
AND
B. E. J. WHEELER
Imperial College Field Station, Silwood Park, Sunninghill, Berks. (With
1
Text-figure)
The development of Sphaerotheca pannosa (Wallr.) Lev. on detached rose leaves was retarded by a water spray especially when this was applied immediately after the leaves were inoculated with conidia. Similar numbers of conidia germinated on wetted and non-wetted leaves but fewer germinated conidia grew to sporulating colonies on mist-sprayed leaves and more remained at the single germ-tube stage suggesting that water affected particularly the initial penetration of the host.
It is well established that water inhibits germination of powdery mildew conidia (e.g. Corner, I93S; Yarwood, 1936; Longree, 1939; Brodie & Neufeld, 1942; Brodie, 1945; Delp, 1954; Schnathorst, 1960; Manners & Hossain, 1963; Morrison, 1964; Zaracovitis, 1964). There are reports also which indicate that when water is applied to plants mildew growth is impaired. Yarwood (1936), for example, found that mildew on peas was destroyed by spraying infected leaves and later reported that water sprays controlled powdery mildews on rose, bean, cucumber and barley (Yarwood, 1939). Though spraying with water does not appear to be commercially satisfactory for the control of rose powdery mildew (McClellan, 1942), information on the ways in which water limits mildew is important in relation to epidemiological studies of the disease (Rogers, 1959). The effect of water on the development of Sphaerotheca pannosa on rose leaves, particularly in its early stages, was investigated in the experiments described below. MATERIALS AND METHODS
The miniature rose cultivar, Perle d'Alcanada was grown in John Innes no. 1 compost, singly in pots (IS'5 cm diam) in the open. Every 10 days, five to eight plants were severely pruned and transferred to a glasshouse at 17-19 °C with a 16 h photoperiod to encourage the production of new shoots. Leaves which had just expanded were used in the experiments and they were inoculated with conidia of S. pannosa as described by Price (1970). The conidia were obtained from cultures on the adaxial surface of young rose leaves, cv. Little Buckeroo, which were floated on distilled water in polystyrene boxes (5'5 x 3'5 x 2'0 cm) and incubated in a cabinet at 17 ± 1 °C with a 16 h photoperiod.
*
Present address: Botany Department, University of Ceylon, Colombo 3.
314
Transactions British Mycological Society
Inoculated leaves were placed in polystyrene boxes (17,8 x 10'2 x 3,8 em) on wet tissue and incubated at 20 ± I "C. Where required inoculated leaves were covered with many minute water droplets by spraying them with a fine mist of distilled water from a Shandon spray gun held about 60-90 em away. At the end of the required wet period leaves were dried under a fan placed 120-150 em away for c. 30 min and were then reincubated in the moist chamber. Growth of S. pannosa on the leaves was assessed as a growth index from celloidin strips stained in cotton-blue-lactophenol: h . d growt
III
sum of grades 100 ex = no. conidia assessed x highest rating (=4)'
There were five grades based on the stages in the development of the fungus over 72 h as indicated by Price (1970): 0, ungerminated conidium; I, single germ-tube and appressorium; 2, appearance of second germtube; 3, appearance of third germ-tube, extension of first and second germ-tubes; 4, branching of germ tubes, production of conidiophore initials and sporulation. Assessments were based on three leaf samples from each treatment. Normally three to four celloidin strips were made of each sample and a total of one hundred conidia were assessed at random on these strips. Treatments were completely randomized within experiments and differences between treatment means were evaluated using Duncan's new multiple range test. EXPERIMENTAL
Expt I. Leaves were sprayed with water immediately after inoculation with conidia of S, pannosa and were kept wet for 2, 4, 6 or 8 h. Growth of the fungus on these leaves was compared with that on leaves not wetted. In this experiment growth was assessed both as an index and as total mycelial growth per 100 conidia. For the latter a tracing was made with a camera lucida of the germ-tube length of each conidium which was graded. The total length was then measured with an opisometer and related to a scale drawn at the same magnification from a stage micrometer. Both methods demonstrated that mildew development was adversely affected by wetting, the extent of this being related to the duration of the wet period (Fig. I). In the later stages, however, differences in growth were better expressed in terms of total mycelial length. This reflected more closely the marked visual differences in mildew growth on leaves from these treatments after 7 days. The relationship between the two measures of growth for the various samples of this experiment is given by the equation: y = 0'I4I4x- 1.8846 (correlation coefficient, 0'97) where x = growth indexand j = loge mycelial length. Expt 2. Leaves were subjected to wet periods of 4, 6 or 8 h either immediately following inoculation with conidia or 6 h later. The results (Table I) confirmed that wet periods of 6 and 8 h following inoculation markedly reduced mildew growth. Wetting leaves 6 h after inoculation with conidia appeared to have a less drastic effect.
Sphaerotheca pannosa. R. G. Perera and B. E.]. Wheeler
315
80
""' 8
-S
.c ~
8tlIl 60
ii
~
~
40
6
24
48
72
Time of incubation (h) Fig.
I.
Growth of Sphaerotheca pannosa on leaves kept wet for varying periods after inoculation .
Expt 3. Leaves were subjected to a wet period of 8 h either immediately after inoculation or 6 h later. Growth at 24, 48 and 72 h was significantly reduced by the former treatment (Table 2a). Wetting leaves 6 h after inoculation appeared only to lead to a temporary check (at 48 h) and by 7'2 h there was no significant difference in growth between this treatment and the control. An analysis of the growth stages reached by conidia (T able '2 b) showed that even on non-wetted leaves some 25 % of the
Transactions British Mycological Society Table I. Growth after 48 h of Sphaerotheca pannosa on leaves kept wet for varying periods immediately after inoculation (A) or 6 h later (B) Growth index
Table
Duration of wet period (h)
Control
A
B
4 6 8
44'4 48'1 56'5
45'0 33'5 21'5
45'3 44,6 30'6
Growth of Sphaerotheca pannosa on leaves given an 8 h wet period immediately after inoculation (A) or 6 h later (B)
2.
(a) Overall growth Incubation (h)
Mean growth index A
\
A
Treatment
6,6
B
Control
7'2 25'5 37'3 47'8
7'7 23'8 47'6 5°'7
Means within one assessment not underscored by the same (or any) line differ significantly (P < 0'05), (b) Growth stages reached Mean no, conidia in each grade Incubation (h) 24
A
Grade ° 2 ° 2 3 4 ° 2 3 4
Treatment
.,
,
A
B
Control
25'3 73'0 1'7
22'0 58'0 20'0
24'0 56'7 19'3
26'3 5°'7 2'7 II'3 g'o 27'0 56'0 1'7 3'0 12'3
22'3 47'0 5"7 g'o 16'0 24'7 29'3 5'7 10'7 29'7
21'3 30'0 8'3 17'7 22'7 25'0 28'7 1'7 7"7 37'0
Means within one grade not underscored by the same line differ significantly (P < 0'05)'
conidia never germinated and about the same number formed one germtube only after 72 h. Only 37 % of the total conidial population grew to a sporulating colony. Wetting leaves immediately after inoculation reduced the number of sporulating colonies and correspondingly, increased the number of conidia which developed only one germ-tube. The check in overall growth at 48 h on leaves wetted 6 h after inoculation stemmed from similar effects but some conidia at the single germ-tube stage then developed further.
Sphaerotheca pannosa. R. G. Perera and B. E.J. Wheeler
317
Table 3. Growth of Sphaerotheca pannosa on leaoes subjected to
different wetting treatments Growth index" after Treatment
\
2h
24 h
36 h
48 h
72 h
96 h
19'0 18'5 20'6 15'0 20'9
25"6 21'7 29'5 22'3 23'0
49'0 4 1'7 40'0 22'8 33'7
53'4 43'4 44,8 28'9 32'9
58 '0 5 1'5 5°'0 36'8 46 '0
56 '5 55'0 58'7
!
No wetting 8 h wet period, 12 h after inoculation 8 h wet period, 24 h after inoculation 12 h wet period immediately after inoculation 12 h wet period , 12 h after inoculation
• Mean of three replicates,
Table 4. Growth of Sphaerotheca pannosa on leaoes with water
droplets for 8 h after inoculation Growth index after 72 h , In areas with water droplets In non-wetted for 8h areas I
Leaf no, I
2
3° '0 21'4 34'2
3 4
41' 1
5 Mean
45'8 34'5
64'0 79'4 78 '9 86,8 67'8 75'4
Expt 4. Leaves were given an 8 h wet period at 12 and 24 h after inoculation. In these treatments growth was slightly less than the controls up to 72 h (T able 3) but by 96 h even these slight differences were no longer apparent. Expt 5. Leaves were kept wet for 12 h immediately after inoculation or 12 h later. Mildew growth was reduced by both treatments (Table 3) but particularly so where wetting immediately followed inoculation. This experiment was set up at the same time as Expt 4 and the control treatment was common to both. Expt 6. Six 2 mm diam rings were marked with Indian ink on the terminalleaflets of each of six leaves and when the ink dried the leaves were inoculated with conidia. Drops of distilled water were placed within three of the marked rings on each leaflet using an Agla syringe, the remaining marked areas were left dry. The leaflets were incubated for 8 h in a polystyrene box lined with wet tissue at 20 DC and then were air dried and immediately re-incubated. Celloidin peels of the marked areas were made 64 h later. The results (Table 4) indicated that mildew growth in the wetted areas was only about half that of the controls. DISCUSSION
The results indicate that the development of powdery mildew on detached rose leaves was retarded by spraying these with water. The effect was most marked and persistent when leaves were wetted imme-
318
Transactions British Mycological Society
diately after inoculation with conidia and the degree to which growth was retarded was directly related to the length of the wet period. Leaf wetness did not appear to inhibit the production of the first germtube by the conidium. On the contrary, germination on mist-sprayed leaves was only marginally less than on dry leaves during the first 6 h. It did, however, reduce the number of germinated conidia that grew to a sporulating colony and increased the number that remained at the r-germ-tube stage. Evidently the most susceptible phase occurs some 6-8 h after inoculation when processes leading to penetration of the host tissue are initiated. Possibly conidia are held above the leaf on the surfaces of the water droplets and the fungus is thus prevented from establishing contact with the host. Certainly in experiments where leaves were sprayed 6 or 12 h after inoculation with conidia, the inhibitory effect on growth was less marked. This suggests that fungal development following initial penetration of the host is much less sensitive to water, though here results must be interpreted with some caution because above values of 35 small differences in growth indices correspond to substantial differences in mycelial extension. Periods ofleafwetness following the initial penetration of the leaf could well have a considerable effect in delaying sporulation of the colony. Even water sprays applied immediately after inoculation with conidia did not completely inhibit mildew development. In this instance it is likely that colonies developed mainly from conidia which were between water droplets. In the field a similar situation could arise during periods of rain for the very young rose leaves which are most susceptible to mildew are those most difficult to wet and on these water stays in discrete droplets. REFERENCES
BRODIE, H.]. (1945). Further observations of the mechanism of germination of the conidia of various species of powdery mildew at low humidity. Canadian Journal of Research C 23, 198-2 I I. BRODIE. H.J. & NEUFELD. C. C. (1942). The development and structure of the conidia of Erysiphe polygoni DC. and their germination at low humidity. Canadian Journal of Research C 20, 41-62. CORNER, E.]. H. (1935). Observations on resistance to powdery mildews. New Phytologist 34, 180-200. DELP, C. L. (1954). Effect of temperature and humidity on the grape powdery mildew fungus. Phytopathology 44,615-626. LONGREE, K. (1939). The effect of temperature and relative humidity on the powdery mildew of roses. Cornell Agricultural Experiment Station Memoir no. 223,43 pp. MANNERS, J. G. & HOSSAIN, S. M. M. (1963). Effect of temperature and humidity on conidial germination in Erysiphe graminis. Transactions of the British Mycological Society 46, 225-234. MCCLELLAN, W. D. (1942). Control of powdery mildew of roses in the greenhouse. Carnell University Agricultural Experiment Station Bulletin no. 785, 39 pp. MORRISON, R. M. (1964). Germination of conidia of Erysiphe cichoracearum. Mycologia 56, 232-236. PRICE, T. V. (1970). Epidemiology and control of powdery mildew (Sphaerotheca pannasa) on rose. Annalsof AppliedBiology 65,231-248. ROGERS, M. N. (1959). Some effects of moisture and host plant susceptibility on the development of powdery mildew of roses caused by Sphaerotheca pannosa var, rosae. Carnell Agricultural Experiment Station Memoir no. 363, 38 pp.
Sphaerotheca pannosa. R. G. Perera and B. E.J. Wheeler 319
w. C. (1960). Effect of temperature and moisture stress on the lettuce powdery mildew fungus. Phytopathology 50, 304-308. YARWOOD, C. E. (1936). The tolerance of Erysiphe polygoni and certain other powdery mildews to low humidity. Phytopathology 26, 845-859. YARWOOD, C. E. (1939). Control of powdery mildews with a water spray. Phytopathology 29, 288-29° . ZARACOVITIS, C. (1964). Factors in testing fungicides against powdery mildews. The germination of conidia in vitro. Annals ofthe Phytopathologual Institute, Benaki N.5. 6, 73- 106. SCHNATHORST,
(Accepted for publication
21
I I
July 1974)
MYC
64