Parasitism of the root-lesion nematode Pratylenchus thornei on chickpea

Pratylenchus thornei -chickpea interactions were investigated under controlled and fluctuating environmental conditions in the growth chamber, greenhouse and shadehouse. Under controlled conditions, P. thornei infected chickpea lines 12071/10054 and P2245 and cultivars Andoum 1, JG62 and UC27. Line P 2245 and cv. JG 62 were the most susceptible genotypes on the basis of root damage and nematode reproduction, but nematode infection did not significantly reduce root and shoot weights. Cultivars Andoum 1 and UC27 and line 12071/10054 showed the least root damage and nematode reproduction. Inoculation of cv. Andoum 1 with 2500, 5000 or 10000 nematodes per plant in pots did not affect shoot weight, regardless of the conditions of water stress of the plants. However, root weight was significantly reduced by nematode infection in plants grown under water stress and fluctuating temperature conditions in the greenhouse, but was not affected by any other treatment. The nematode reproduction index was not affected by soil water content under shadehouse conditions, but was greater on plants watered to soil water-holding capacity than in water-stressed plants under greenhouse conditions. For both environments, the nematode reproduction index decreased when inoculum density was greater than 5000 nematodes per plant.     

Pathogenicity and histopathology of Pratylenchus thornei populations on selected chickpea genotypes

Four populations of Pratylenchus thornei from different locations were tested for reproductive fitness in axenic carrot disc cultures and for pathogenicity to chickpea cultivars JG 62 and UC 27 and lines K 850 and ILC 1929. Parasitism and histopathology on selected chickpea genotypes (JG 62, UC 27 and lines ILC 482, ICC 11324 and ICC 12237) were also investigated. Reproductive fitness, assessed as the ratio of the final number of nematodes per carrot disc to the number of nematodes inoculated, was similar among the populations tested and the four populations reproduced to a similar extent in a given chickpea genotype. However, the extent of reproduction was significantly affected by the chickpea genotype, JG 62 and UC 27 being the best and poorest hosts, respectively. Pathogenicity to chickpea genotypes was assessed by the difference in fresh root and dry shoot weights between infected and uninfected plants 90 days after inoculation. Plant growth was significantly reduced by the four nematode populations in all chickpea genotypes, with the exception of cv. JG 62, which was tolerant of P. thornei . Severity of root necrosis caused by nematode infection was similar for all populations. Histopathological studies of chickpea genotypes infected by P. thornei showed that all were suitable hosts according to nematode reproduction and host reaction. P. thornei always migrated through epidermal and cortical cells by breaking down cell walls along the nematode pathway. In the most susceptible lines (ILC 482 and JG 62), damage to endodermal cells adjacent to nematode feeding sites was occasionally observed.     

Interactions of Pratylenchus thornei and Fusarium oxysporum f. sp. ciceris on Chickpea

ABSTRACT Fusarium oxysporum f. sp. ciceris and the root-lesion nematode Pratylenchus thornei coinfect chickpeas in southern Spain. The influence of root infection by P. thornei on the reaction of Fusarium wilt-susceptible (CPS 1 and PV 61) and wilt-resistant (UC 27) chickpea cultivars to F. oxysporum f. sp. ciceris race 5 was investigated under controlled and field conditions. Severity of Fusarium wilt was not modified by coinfection of chickpeas by P. thornei and F. oxysporum f. sp. ciceris, in simultaneous or sequential inoculations with the pathogens. Root infection with five nematodes per cm(3) of soil and 5,000 chlamydospores per g of soil of the fungus resulted in significantly higher numbers of propagules of F. oxysporum f. sp. ciceris with the wilt-susceptible cultivar CPS 1, but not with the wilt-resistant one. However, infection with 10 nematodes per cm(3) of soil significantly increased root infection by F. oxysporum f. sp. ciceris in both cultivars, irrespective of fungal inoculum densities (250 to 2,000 chlamydospores per g of soil). Plant growth was significantly reduced by P. thornei infection on wilt-susceptible and wilt-resistant chickpeas in controlled and field conditions, except when shorter periods of incubation (45 days after inoculation) were used under controlled conditions. Severity of root necrosis was greater in wilt-susceptible and wilt-resistant cultivars when nematodes were present in the root, irrespective of length of incubation time (45 to 90 days), densities of nematodes (5 and 10 nematodes per cm(3) of soil), fungal inocula, and experimental conditions. Nematode reproduction on the wilt-susceptible cultivars, but not on the wilt-resistant one, was significantly increased by F. oxysporum f. sp. ciceris infections under controlled and field conditions.     

The effect of inoculation temperature on the reaction of potato cultivars to Synchytrium endobioticum

The reactions of potato cultivars to inoculation with germinating winter spores of Synchytriuum endobioticum at 10, 15 and 20°C were compared. The classification of wart reactions for cultivars at the Resistant Grade 1/Resistant Grade 2 (RG1/RG2) and RG2/susceptible borderlines was found to depend on the inoculation temperature. For RG2 cultivars, there was a clear increase in the proportion of more resistant reactions with increase in inoculation temperature. The borderline cultivars 'Teena' and 'Alhamra' reacted as RG2 when inoculated at 10°C and as RG1 when inoculated at higher temperatures. However the inoculation temperature is unlikely to influence the outcome of tests on cultivars that produce a higher percentage of RG2 reactions. An inoculation temperature of 10–15°C more readily identified cultivars that are slightly susceptible. The slightly susceptible cultivars 'Apache', 'Morven' and 'Hassia' reacted as susceptible at 10 and 15°C and as RG2 at 20°C. However, cultivars known to produce wart tissue, including the very slightly susceptible 'Altena', were found readily to produce susceptible reactions when inoculated at all the temperatures tested.     

Formation and survival of oospores of Phytophthora infestans under natural conditions

Phytophthora infestans is able to produce oospores in leaves of potato and tomato plants after inoculation with a mixture of Al and A2 mating-type isolates. Various conditions for oospore formation were analysed. Under controlled conditions, oospores were produced in potato leaves at temperatures ranging from 5 to 25° C. In leaves of potato cultivar Bintje incubated at 15°C, oogonia and antheridia were observed 6 days after inoculation and thick-walled oospores appeared 3-4 days later. In field experiments oospores were found in leaves and stems of potato cultivars Bintje, Irene and Pimpernel and in leaves, stems and fruits of tomato cultivar Moneymaker within 2 weeks after inoculation. A bioassay was developed to test the survival of oospores in soil under various conditions. To determine whether late-blight infections derived from infectious soil were caused by oospwres, DNA fingerprinting was performed. DNA fingerprint probe RG-57 was suitable for distinguishing asexual progeny from recombinant progeny arising from soil-borne oospores. We demonstrated survival of viable, infectious oospores of P. infestans in soil during the winter of 1992–93. Oospores were not infectious from soil exposed to temperatures of 40°C or higher but in the range 35°C to as low as – 80°C for 48 h, oospores survived.     

Effects of temperature and wetness duration on conidial infection, latent period and asexual sporulation of Pyrenopeziza brassicae on leaves of oilseed rape

Experiments in controlled environments were carried out to determine the effects of temperature and leaf wetness duration on infection of oilseed rape leaves by conidia of the light leaf spot pathogen, Pyrenopeziza brassicae . Visible spore pustules developed on leaves of cv. Bristol inoculated with P. brassicae conidia at temperatures from 4 to 20°C, but not at 24°C; spore pustules developed when the leaf wetness duration after inoculation was longer than or equal to approximately 6 h at 12–20°C, 10 h at 8°C, 16 h at 6°C or 24 h at 4°C. On leaves of cvs. Capricorn or Cobra, light leaf spot symptoms developed at 8 and 16°C when the leaf wetness duration after inoculation was greater than 3 or 24 h, respectively. The latent period (the time period from inoculation to first spore pustules) of P. brassicae on cv. Bristol was, on average, approximately 10 days at 16°C when leaf wetness duration was 24 h, and increased to approximately 12 days as temperature increased to 20°C and to 26 days as temperature decreased to 4°C. At 8°C, an increase in leaf wetness duration from 10 to 72 h decreased the latent period from approximately 25 to 16 days; at 6°C, an increase in leaf wetness duration from 16 to 72 h decreased the latent period from approximately 23 to 17 days. The numbers of conidia produced were greatest at 12–16°C, and decreased as temperature decreased to 8°C or increased to 20°C. At temperatures from 8 to 20°C, an increase in leaf wetness duration from 6 to 24 h increased the production of conidia. There were linear relationships between the number of conidia produced on a leaf and the proportion of the leaf area covered by 'lesions' (both log₁₀-transformed) at different temperatures.     

STUDIES ON HERBICIDE CONTENTS IN ROOTS OF SKELETON WEED (CHONDRILLA JUNCEA L.) FOLLOWING LEAF APPLICATIONS

Summary. The herbicides studied were 2,4-D, 2,4-DB, dicamba and orthoarsenic acid. Herbicide content in the roots was taken as an overall measure of penetration into and absorption by the leaves, and of translocation to the roots.
A significantly greater 2,4-D content resulted from foliar application at pH 3–5 than at higher values, though at pH 8–5 the inclusion of triethanolamine significantly increased the 2,4-D content. No evidence was obtained that a greater 2,4-D content should result from foliage applications of 2,4-DB than from 2,4-D. Dicamba gave a greater herbicide content than 2,4-D when applied at high concentration at 20° C but not at 25° C, probably because of less injury at the lower temperature.
Concentrations of Tween 20 up to 2% had no deleterious effect on the 2,4-D content; on the other hand 2,4-D content was lowered by 0–25% or more of cetyltrimethyl-ammonium bromide. Poor wetting is not the cause of the variable herbicide contents sometimes obtained.
Orthoarsenic acid, which has given better control of the weed than 2,4-D, was very poorly translocated; its effectiveness is due to its high intrinsic toxicity.
Etudes sur la teneur en herbicide des racines de Chondrilla juncea L. á la suite d'applications sur les feuilles     

Interaction of root-lesion nematodes, Pratylenchus thornei and P. crenatus, with the fungus Thielaviopsis basicola and a grey sterile fungus on roots of pea (Pisum sativum)

Pratylenchus thornei invaded excised pea roots in agar in greater numbers and penetrated the cortex more deeply than P. crenatus . Both species fed on the roots ectoparasitically and displaced root cells into the surrounding medium. The cytoplasm of cortical cells near cither nematode became granulated, with enlarged vacuoles and nuclei. P. thornei also caused these responses in the endodermis. Infection of the root surface with a grey sterile fungus inhibited invasion by P. crenatus and P. thornei . Infection by Thielaviopsis basicola inhibited P. thornei invasion but encouraged penetration by P. crenatus and the hyphae were found deeper in the cortex when P. crenatus was present.     

In vitro effects of temperature and wet periods on infection of oilseed rape by Alternaria brassicae

The influence of temperature and duration of wet periods on infection of oilseed rape by Alternaria brassicae was studied on detached leaves and pods, leaf disks and intact seedlings. Infections increased with age of leaf and the interaction between temperature and leaf age was highly significant. On older leaves infection was optimal at 25°C. There were many infections also at 15, 20 and 29 C but relatively few infections at 10°C. On pods most infections were observed at 20 C, the highest temperature studied. Infection at each temperature increased progressively with duration of surface wetness. The minimum wet periods for infection of leaves were 3 h at 20–25°C, 4 h at 15°C, 6–9 h at 10 C and 12–24 h at 5 C and for infection of pods, between 6 h and 9 h at 10°C and 6 h (or less) at 15°C and 20 C. On leaves, dry periods interrupting wet periods limited lesion development to that obtained with the initial wet period only; on pods some further infections developed when pods were re-wetted. Dry periods of 3 h and 6 h following the inoculation of pods reduced subsequent infection but there was no further reduction by longer periods of drying to 48 h.     

Environmental conditions influencing Sclerotinia sclerotiorum infection and disease development in lettuce

The environmental factors that influence infection of lettuce by ascospores of Sclerotinia sclerotiorum , and subsequent disease development, were investigated in controlled environment and field conditions. When lettuce plants were inoculated with a suspension of ascospores in water or with dry ascospores and exposed to a range of wetness durations or relative humidities at different temperatures, all plants developed disease but there was no relationship between leaf wetness duration or humidity and percentage of diseased plants. Ascospores started to germinate on lettuce leaves after 2–4 h of continuous leaf wetness at optimum temperatures of 15–25°C. The rate of development of sclerotinia disease and the final percentage of plants affected after 50 days were greatest at 16–27°C, with disease symptoms first observed 7–9 days after inoculation, and maximum final disease levels of 96%. At lower temperatures, 8–11°C, disease was first observed 20–26 days after inoculation, with maximum final disease levels of 10%. Disease symptoms were always observed first at the stem base. In field-grown lettuce in Norfolk, 2000 and 2001, inoculated with ascospore suspensions, disease occurred only in lettuce planted in May and June, with a range of 20–49% of plants with disease by 8 weeks after inoculation. In naturally infected field-grown lettuce in Cheshire, 2000, disease occurred mainly in lettuce planted throughout May, with a maximum of 31% lettuce diseased within one planting, but subsequent plantings had little (≤ 4%) or no disease. Lack of disease in the later plantings in both Norfolk and Cheshire could not be attributed to differences in weather factors.     

Influence of temperature on plant–rhizobacteria interactions related to biocontrol potential for suppression of fusarium wilt of chickpea

Seed and soil treatment with Pseudomonas fluorescens RGAF 19, P. fluorescens RG 26, Bacillus megaterium RGAF 51 and Paenibacillus macerans RGAF 101 can suppress fusarium wilt of chickpea ( Cicer arietinum ), but the extent of disease suppression by these rhizobacteria is modulated by soil temperature. In this work, the effect of temperature on plant–rhizobacteria interactions was assessed in relation to biocontrol potential for suppression of fusarium wilt of chickpea. Seed and soil treatment with those rhizobacteria delayed seedling emergence compared with nontreated controls, and either increased or had no deleterious effect on chickpea growth. Pseudomonas fluorescens isolates significantly increased chickpea shoot dry weight at 20°C and root dry weight at 25 and 30°C. All bacterial isolates colonized the chickpea rhizosphere and internal stem tissues at 20, 25 and 30°C, and there was a positive linear trend between bacterial population size in the rhizosphere and temperature increase. The maximum inhibition of mycelial growth and conidial germination of Fusarium oxysporum f. sp. ciceris race 5 in vitro occurred at a temperature range optimal for bacterial growth and production of inhibitory metabolites. These results demonstrate the need to understand the effects of environmental factors on the biological activities of introduced rhizobacteria of significant importance for plant disease suppression.     

Infection of ryegrass by three rust fungi (Puccinia coronata, P. graminis and P. loliina) and some effects of temperature on the establishment of the disease and sporulation

Experiments were conducted to examine the processes leading up to the infection of Lolium temulentum by crown rust ( Puccinia coronata ), stem rust ( P. graminis ) and brown rust ( P. loliina ), and the effects of temperature on these processes and sporulation. Uredia of all three rusts were produced freely if the adaxial leaf surface was inoculated, but did not form following inoculation of the abaxial surface. Light and scanning electron microscopy revealed abnormal growth of germlings on the abaxial surface which had amorphous sheet-like epicuticular waxes and very few stomata. On the adaxial leaf surface germ tubes of all the rusts orientated at right angles to the long axis of the leaf. However, the directional growth of germ tubes was often disrupted when they contacted the surface of bulliform cells at the base of leaf grooves. For P. loliina the optimum temperatures for urediospore germination and sub-stomatal vesicle formation were 12–16°C, and 8–20°C for appressorium formation. The optimum temperatures, for the same stages of fungal development, for P. coronata and P. graminis were higher. Urediospore production of P. loliina was higher at 10°C than at 25°C, but was similar at both temperatures for P. coronata .     

Sporulation of Pyrenopeziza brassicae (light leaf spot) on oilseed rape (Brassica napus) leaves inoculated with ascospores or conidia at different temperatures and wetness durations

In controlled environment experiments, sporulation of Pyrenopeziza brassicae was observed on leaves of oilseed rape inoculated with ascospores or conidia at temperatures from 8 to 20°C at all leaf wetness durations from 6 to 72 h, except after 6 h leaf wetness duration at 8°C. The shortest times from inoculation to first observed sporulation ( l ₀), for both ascospore and conidial inoculum, were 11–12 days at 16°C after 48 h wetness duration. For both ascospore and conidial inoculum (48 h wetness duration), the number of conidia produced per cm² leaf area with sporulation was seven to eight times less at 20°C than at 8, 12 or 16°C. Values of Gompertz parameters c (maximum percentage leaf area with sporulation), r (maximum rate of increase in percentage leaf area with sporulation) and l ₃₇ (days from inoculation to 37% of maximum sporulation), estimated by fitting the equation to the observed data, were linearly related to values predicted by inserting temperature and wetness duration treatment values into existing equations. The observed data were fitted better by logistic equations than by Gompertz equations (which overestimated at low temperatures). For both ascospore and conidial inoculum, the latent period derived from the logistic equation (days from inoculation to 50% of maximum sporulation, l ₅₀) of P. brassicae was generally shortest at 16°C, and increased as temperature increased to 20°C or decreased to 8°C. Minimum numbers of spores needed to produce sporulation on leaves were ≈25 ascospores per leaf and ≈700 conidia per leaf, at 16°C after 48 h leaf wetness duration.     

Factors affecting mummification and sporulation of pome fruit infected by Monilinia fructigena in Dutch orchards

A 2-year field experiment (1997–98, 1998–99) was conducted to study mummification and subsequent sporulation in spring of apple (cvs James Grieve, Golden Delicious) and pear (cv . Conference) fruits infected by Monilinia fructigena . Most mummified fruits were found in James Grieve and Conference, whereas in late-infected Golden Delicious, fruits were still soft when examined in April. In the first year, these late-infected fruits had a significantly higher sporulation intensity per sporulating fruit ( P  = 0·05) compared with Golden Delicious fruits infected 9 and 5 weeks before harvest maturity, which were partly mummified. It was concluded that early- and late-infected fruits contributed to primary inoculum in the next season. In a postinfection regime of 25°C and 65–75% relative humidity under controlled conditions, the number of Conference fruits sporulating decreased rapidly, and after 12 weeks' incubation sporulation had completely ceased. After 8 weeks' incubation, sporulation intensity in the postinfection regime at 10°C was significantly higher than that at 20 and 25°C in a first experiment with inoculated unripe fruit ( P  = 0·05). Results of a second experiment with ripe fruit were less clear. These results are discussed in relation to orchard disease management.     

Effect of temperature on head blight of wheat caused by Fusarium culmorum and F. graminearum

The effect of small temperature differentials (16 vs. 20°C) on the pathogenicity of deoxynivalenol producing single isolates of Fusarium culmorum and F. graminearum and on the fusarium head blight (FHB) response of eight wheat cultivars was examined. Fusarium culmorum inoculation caused greater visual disease symptoms at 20°C than at 16°C, both overall and on an individual cultivar basis (overall AUDPC = 13·5 and 9·6, respectively) ( P  < 0·05). In contrast, F. graminearum inoculation caused greater overall visual disease symptoms at 16°C than at 20°C, both overall and at the individual cultivar level (overall AUDPC = 12·8 and 10·9, respectively) ( P  < 0·05). Results showed both F. culmorum and F. graminearum inoculations caused a greater loss in yield at 20°C (54·3 and 46·9% relative 1000-grain weight, respectively) compared with 16°C (73·3 and 66·9% relative 1000-grain weight, respectively) ( P  < 0·05). Fusarium culmorum -inoculated heads contained similar amounts of fungal DNA at both 16 and 20°C (1·9 and 1·7 ng mg⁻¹ of plant material, respectively) (not significant), while for F. graminearum inoculation, plants contained higher amounts of fungal DNA at 20°C (2·0 and 1·0 ng mg⁻¹ of plant material, respectively) ( P  < 0·05). Overall, there was a significant negative correlation between AUDPC and percentage relative 1000-grain weight at both 16 and 20°C ( r  =−0·693 and −0·794, respectively, P  < 0·01).     

Eradication of Plasmodiophora brassicae during composting of wastes

Survival of infectious inoculum of the clubroot pathogen Plasmodiophora brassicae was assessed following bench-scale flask composting experiments and large-scale composting procedures. Clubroot-affected material was provided by artificial inoculation of Chinese cabbage or naturally infected Brussels sprout and cabbage roots. Both sets of diseased material were used in flask experiments, and the latter in large-scale windrow and aerated tunnel experiments. Municipal green wastes, onion waste and spent mushroom compost were evaluated in flask experiments with varying temperature, aeration and moisture conditions. Green wastes were used in larger-scale composts. Within the limits of a Chinese cabbage seedling bioassay, both temperature and moisture content were critical for eradication of P. brassicae spores extracted from composted clubroot-affected residues. Incubation in compost at 50°C for 7 days or 1 day at 60°C with high moisture levels (= −5 kPa matric potential or 60% w/w moisture content) eradicated inoculum from artificially inoculated Chinese cabbage roots. In large-scale windrows and aerated tunnels, the pathogen was eradicated from naturally infected brassica wastes after 6–7 days at 54–73°C.     

Infection of onion leaves by Alternaria porri and Stemphylium vesicarium and disease development in controlled environments

Infection of onion by Alternaria porri and Stemphylium vesicarium was investigated under a range of controlled temperatures (4–25°C) and leaf wetness periods (0–24 h). Conidia of A. porri and S. vesicarium germinated within 2 h when incubated at 4°C. Terminal and intercalary appressoria were produced at similar frequencies at or above 10°C. The maximum number of appressoria was produced after 24 h at 25°C. Penetration of leaves by both pathogens was via the epidermis and stomata, but the frequency of stomatal penetration exceeded that of epidermal penetration. There was a strong correlation ( R ² > 90%) between appressorium formation and total penetrations at all temperatures. Infection of onion leaves occurred after 16 h of leaf wetness at 15°C and 8 h of leaf wetness at 10–25°C, and infection increased with increasing leaf wetness duration to 24 h at all temperatures. Interruption of a single or double leaf wetness period by a dry period of 4–24 h had little effect on lesion numbers. Conidia of A. porri and S. vesicarium separately or in mixtures caused similar numbers of lesions. Alternaria porri and S. vesicarium are both potentially important pathogens in winter-grown Allium crops and purple leaf blotch symptoms were considered to be a complex caused by both pathogens.     

Latent infection of winter oilseed rape by Leptosphaeria maculans

Plants of oilseed rape, cultivars Primer and Jet Neuf, were grown in a glasshouse and inoculated at G.S. 2.4–2.7 with pycnidiospores or ascospores of Leptosphaeria maculans. The plants were kept for a further 2–4 weeks at 14°C and then transferred, together with uninoculated plants, to a polythene tunnel in winter. The majority of stems of inoculated plants did not have macroscopic symptoms of L. maculans infection 6 weeks after inoculation. Examination of whole mounts of peripheral tissue and transverse sections of fixed and embedded portions of these stems revealed intercellular septate fungal hyphae, often deep in non-necrotic cortical tissue, in symptomless inoculated plants but not in uninoculated plants. L. maculans was recovered following surface sterilization of adjacent portions of the same stems. When symptomless inoculated plants were transferred to a glasshouse at 18–20°C, cankers soon developed. The significance of these latent mycelial infections to canker development in the field is discussed.     

Effects of bean line pattern mosaic virus on the monocyclic components of rust and angular leaf spot of Phaseolus bean at different temperatures

Monocyclic components (development rate during the incubation period or latent period, lesion density, lesion size and disease severity) of rust ( Uromyces appendiculatus ) and of angular leaf spot ( Phaeoisariopsis griseola ) in two bean ( Phaseolus vulgaris ) cultivars (Rosinha G-2 and Carioca), pre-infected or not with bean line pattern mosaic virus (BLPMV), were determined. Trials were conducted at temperatures in the range from 9 to 27°C for rust and from 12 to 30°C for angular leaf spot. Regardless of viral pre-infection, the effect of temperature on the four monocyclic components followed an optimum curve and could be described by a generalized beta function. Generally, angular leaf spot was favoured by higher temperatures with an optimum for disease severity between 24.2 and 28.3°C compared with 15.9–18.5°C for rust. Pre-infection with BLPMV did not change the shape of the optimum curves for all components, but significantly reduced lesion density and disease severity on both cultivars. The development rates during incubation and latent periods for both fungal diseases were not affected by BLPMV. Pre-infection with virus did not alter the ranking of cultivars with respect to resistance to both fungal diseases.     

Germination of Alternaria linicola conidia on linseed: effects of temperature, incubation time, leaf wetness and light regime

Conidia of Alternaria linicola germinated on both water agar and linseed leaves (detached or attached) over a wide range of temperatures (5–25°C) by producing one to several germ tubes. At temperatures between 10°C and 25°C and under continuous wetness in darkness, germination started within 2 h after inoculation and reached a maximum (100%) by 8 to 24 h, depending on temperature. At 5°C, the onset of germination was later and the rate of germ tube elongation was slower than that at 10–25°C. During germination, conidia of A. linicola were sensitive to dry interruptions of wet periods and to light. Short (2 h) or long (12 h) dry interruptions occurring at any time between 2 and 6 h after inoculation stopped conidial germination and germ tube elongation. With continuous wetness, light periods 2 to 12 h long immediately after inoculation inhibited conidial germination, which was resumed only when a dark period followed subsequently. However, germination and germ tube elongation of A. linicola conidia stopped and the viability of the conidia was lost during exposure to dry light periods immediately after inoculation with spore suspensions. Penetration of leaves by A. linicola was evident after 12 h and occurred mainly through epidermal cells (direct) with or without the formation of appressoria.