Host and environmental effects on the infection of maize by Puccinia sorghi. I. Prepenetration development and peneration

Abstract

Germination of urediniospores of Puccinia sorghi on leaves and on agar was similar over the range 5-25 O C , being greatest at 15 O C . At this temperature, maximum germination was attained within 6 h. Germination on cover slips started at around 99% r.h. and increased with increase of humidity.Urediniospore germination was not affected by leaf age. In general, proportionally more spores germinated on the abaxial than on the adaxial surface. Maximum germination was observed on the abaxial surface of the tip portion of the leaf.The optimum temperature for infection structure formation was about 15 OC. A minimum period of 3-4 h was required for the initiation of infection. Increase in appressorium and sub-stomatal vesicle formation with increase in dew period was observed, with the maxima being attained at about 24 h after inoculation. I N T R O D U C T I O NAs a silage crop, maize is becoming popular with farmers in England. At present, Puccinia sorghi Schw. is not a major problem on maize crops in England, but, with the massive increase in maize cultivation, a severe outbreak cannot be ruled out.Little attention has been paid to P. sorghi in England. Reports of research work on some aspects of this fungus elsewhere in the world have been published in the past (Kushalappa & Hegde, 1971; Mederick & Sackston, 1972). Studies on the effect of temperature have been reported (Kushalappa & Hegde, 1971) but important aspects of biology and development have not been investigated fully and information is lacking on environmental and host factors. Therefore, the purpose of the work reported in this paper was to gather basic information on the growth of the fungus on maize under different environmental and host conditions. M A T E R I A L S A N D M E T H O D S Host cultureSeeds of the maize cultivar Kelvedon 33 were sown in soil-less compost in 11 cm pots and incubated in a glasshouse at 20-25 O C . After germination the pots were transferred to a glasshouse at 15-20 O C and a 16 h photoperiod with daylight supplemented by 400 w highpressure mercury vapour lamps. InoculumMaize leaves bearing uredinia of Puccinia sorghi were first collected from the plots of the National Institute of Agricultural Botany, Seale Hayne, Newton Abbot, Devon. A culture was developed from a single urediniospore as described by Mahindapala (1976). The culture was maintained by transfer of urediniospores, 4 wk after inoculation, to 3-wk old seedlings.

Host and environmental effects on the infection of maize by Puccinia sorghi. II. Post‐penetration development

“…The progressive increase in the number of uredinia which developed and the decrease in the minimum period needed for successful establishment of the pathogen, with rise in temperature, indicate that the optimum temperature for both processes is at least 20 OC. This temperature is higher than the optimum for the processes of urediniospore germination and infection structure formation, both of which have an optimum temperature of 15 "C (Mahindapala, 1978). Previous experiments (Mahindapala, 1978) indicated that at 10 O C and 15 O C , maximum urediniospore germination is attained after 6 h. However, the increase in number of uredinia produced by incubation in excess of 6 h indicates that prolonged incubation at 100% r.h. enhances the number of successful infections.…”

“…This temperature is higher than the optimum for the processes of urediniospore germination and infection structure formation, both of which have an optimum temperature of 15 "C (Mahindapala, 1978). Previous experiments (Mahindapala, 1978) indicated that at 10 O C and 15 O C , maximum urediniospore germination is attained after 6 h. However, the increase in number of uredinia produced by incubation in excess of 6 h indicates that prolonged incubation at 100% r.h. enhances the number of successful infections. This is substantiated by the evidence presented by Mahindapala (1 978) where the number of appressoria and substomatal vesicles continued to increase with the increase of incubation period (at 100% r.h.) up to 24 h. It seems that external water is desirable for the production of appressoria and substomatal vesicles, as well as being required for germ tube growth.…”

“…The isolation of P . sorghi and its maintenance in the glasshouse have been reported earlier (Mahindapala, 1978). Unless otherwise mentioned, plants were inoculated by spraying a urediniospore suspension ( lo5 urediniospores/ml) using a laboratory atomiser.…”

“…These plants were then transferred to a controlled-environment cabinet at the temperature under investigation. At pustule eruption, 25 leaves were labelled and the pustule number assessed using a key (Mahindapala, 1976). Urediniospores were collected every other day from these leaves using a detachable cyclone separator (Tervet, Rawson, Cherry 8c Saxon, 195 1) and weighed.…”

“…Previous work has shown the importance of host and environmental conditions on the germination of urediniospores and subsequent infection of maize by Puccinia sorghi Schw. (Mahindapala, 1978).…”