Effects of humidity and air speed on sporulation of Phytophthora infestans on potato leaves

An apparatus is described for the control of humidity, air speed and temperature around detached potato leaflets and whole plants. Leaflets and plants inoculated with Phytophthora infestans were incubated in air at 15°C. Abundant sporangia were formed in an air speed of 0.3 × 10⁻³ m/s when the ambient humidity was 90–100% RH, but not at 85 or 80% RH. At air speeds of 5.5 × 10⁻³ and 13.7 × 10⁻³ m/s there were numerous sporangia at 100% RH, but not at 95–80% RH. The number of sporangia formed on leaflets kept in air flowing at 5.5 × 10⁻³ m/s with alternating humidities of 80 and 100% RH each for 12 h per day was intermediate between numbers formed on leaflets incubated at each humidity continuously, and higher than numbers formed at a constant 90% RH. Removing leaflets from plants before inoculation did not affect the number of sporangia produced.     

Components of quantitative resistance to powdery mildew (Erysiphe pisi) in pea (Pisum sativum)

Components of quantitative resistance in pea ( Pisum sativum ) to Erysiphe pisi , the pathogen causing powdery mildew, were investigated. Conidium germination, infection efficiency, latent period and conidium production dynamics on cv. Quantum (quantitatively resistant) were compared with those on Pania and Bolero (susceptible). There was an additional comparison in conidium germination experiments with the resistant cv. Resal. Quantitative resistance in Quantum did not affect conidium germination, but infection efficiency of conidia on this cultivar was 34% less than on the susceptible Pania. More conidia germinated on 5-day-old leaflets than on 15-day-old leaflets but the age of the plant did not affect percentage germination or infection efficiency. The length of the latent period did not differ between cultivars. Total conidium production (AUC) per unit leaflet area on Quantum was 25% less than on Pania. The maximum conidium production per day (CMAX) per unit leaflet area on Quantum was 33% less than on Pania. The time to maximum conidium production per day (TMAX) was 10% longer on Quantum than on Pania. The cv. Bolero, reported to be susceptible, also showed some degree of quantitative resistance, but this differed from that of Quantum. Total conidium production was less on Bolero than on Quantum, but the conidia on Bolero were produced sooner, and for a shorter period, than on Quantum. The stability of these responses was tested by analysing components in three different temperature regimes and testing for interactions with temperature, and with leaflet age. Temperature affected all conidium production variables. AUC per leaflet area was nearly seven times as great and CMAX nearly 15 times greater at 23°C than at 13°C. TMAX increased by 1.5 times when temperature increased from 13°C to 18°C or 23°C. Several interactions occurred and these are described.     

Optimizing conditions for growth and sporulation of Pyrenophora semeniperda

The effects of photoperiod, light quality, wounding and sealing of culture plates on in vitro growth and sporulation of Pyrenophora semeniperda were examined to define conditions conducive to the abundant and rapid production of conidia. For maximum growth and sporulation, the leaf-spotting and seedborne plant pathogen required an alternating light/dark sequence. Fewer conidia were produced under conditions of constant illumination or constant darkness. Growth was enhanced by light of wavelengths longer than 500 nm, while sporulation was enhanced by light of wavelengths shorter than 500 nm. Sporulation was enhanced under conditions of alternating temperature with a diurnal photoperiod and by mycelial wounding. Sealing of Petri plates had an inhibitory effect on sporulation. Significantly more conidia of higher quality formed when unsealed cultures, wounded after 7 days, were exposed to 23°C during the light phase and 19°C during darkness. Under these optimal conditions, conidial numbers were increased by 800% or more compared with unwounded cultures grown at constant 25°C in 12 h alternating cool-white light.     

The effect of temperature and light intensity on growth and sporulation of Puccinia striiformis on wheat

Urediniospore production by Puccinia striiformis on wheat per unit leaf area infected was much lower at low light intensities than at high light intensities. The number of pustules per unit area of infected leaf and the daily sporulation rate per pustule increased linearly with increasing light over the range 10–50 W/m². Increasing temperature between 7 and 20°C shortened latent period and reduced the longevity of sporulating leaves. Colonization rate and the frequency of pustules per unit area of infected leaf increased between 7 and 15°C but declined markedly at 20°C. Spore production reached its peak earlier and declined more rapidly with increasing temperature between 7 and 15°C. this decline being less marked in the highly susceptible cultivar Maris Beacon than in the more resistant Maris Nimrod and Maris Huntsman.     

Effects of temperature, leaf wetness and cultivar on the latent period of Mycosphaerella graminicola on winter wheat

After inoculation of winter wheat cv. Longbow at a single time, lesions of M. graminicola were produced over a long interval starting 15–35 days after inoculation, dependent on temperature. There was no evidence that a single infection gave rise to more than one lesion. After the initial infection period at 100% relative humidity (r.h.), keeping leaves wet for c. 10 h per day did not shorten latent period on seedlings. Experiments in controlled-environment chambers demonstrated a minimum latent period at approximately 17°C Variation in the latent period of individual lesions was also minimum at this temperature. The latent period varied among the cultivars tested, cv. Longbow having the shortest, cv. Avalon having almost the longest. Field observations broadly confirmed the results of experiments in constant-environment chambers.     

A recessive resistance gene of alfalfa to alfalfa mosaic virus and impact of high ambient temperature on virus resistance

Twenty plants of alfalfa cv. Beaver were screened for resistance to alfalfa mosaic virus (AIMV) severe strain A-515. ELISA screening on both inoculated and apical leaves at fixed temperature (20°C day, 16°C night) suggested the following three types of clonal response to AIMV infection: extremely resistant, AIMV not normally detected from either inoculated or apical leaves; resistant, AIMV detected from inoculated leaves only and did not spread systemically; susceptible, AIMV detected from both inoculated and apical leaves. When plants in the second category were maintained at high temperature (30°C), AIMV was detected from inoculated and apical leaves 6 and 12 days after inoculation, but was not detectable in the apical leaves thereafter. Plants in the first category remained extremely resistant at all temperatures tested. The results of comparative tests using progeny plants of extremely resistant, resistant and susceptible plants, and of their hybrids, suggested that the resistance to AIMV A-515 was controlled by a temperature-dependent recessive gene.     

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.     

Colonization and sporulation of Peronospora viciae on cultivars of Pisum sativum

The behaviour of Peronospora viciae was examined on four pea ( Pisum sativum ) cultivars (Maro, Superb, Victory Freezer and Surprise) which appeared to exhibit varying degrees of resistance to this pathogen. Penetration into all cultivars was similar and mainly by direct entry through the cuticle. The pathogen grew in a similar manner and at a similar rate in all the cultivars during the early stages (<96 h) of tissue colonization. As it approached the onset of sporulation its growth slowed down in the more resistant cultivars but in no instance were any hypersensitive or other host cell reactions observed. Greater differences between the cultivars were seen in the extent of sporulation, with the least resistant cultivar (Superb) bearing the largest number of sporangiospores. There was much less sporulation on the most resistant cultivar (Maro) and the process was more dependent on temperature than it was on Superb. The development of mature plant resistance occurred in a similar way and on a similar time-scale in all cultivars. The youngest leaflets on plants which were less than 8 weeks old at inoculation showed the least resistance to infection and colonization.     

Effect of light duration on severity of black dot caused by Colletotrichum coccodes on potato

The effect of light duration on potato black dot was examined in plants grown under controlled conditions and in plantlets in tissue culture. A short-day (SD) photoperiod of 8 h light enhanced disease levels caused by Colletotrichum coccodes compared with a long-day (LD) photoperiod of 16 h light. Four cultivars (Agria, Cara, Désirée and Nicola) exhibited higher disease levels under SD conditions. Fungal colonization rates, level of sclerotia on roots, and crown rot symptoms were significantly higher under SD conditions in all cultivars. Parameters related to plant development (height and shoot weight) were reduced in plants grown under SD conditions in all cultivars tested, regardless of inoculation with the fungus. Similar results were also obtained with tissue-culture plantlets of cvs Cara, Désirée, Nicola and Alpha grown in an aseptic system under SD and LD conditions. Fungal colonization indices, level of sclerotia on roots, and crown rot symptoms on inoculated plantlets were significantly more severe in all cultivars tested when grown under SD conditions. Plant height and shoot weight of cvs Cara, Désirée and Nicola were significantly affected by both day length and inoculation with C. coccodes , with inoculated plantlets being shorter and smaller under SD conditions. These findings support the hypothesis that light duration plays an important role in C. coccodes infection, and may be one of the major factors responsible for the severe expression of black dot and yield losses observed in the Israeli autumn season, as may also occur in other potato-production areas in the Mediterranean region.     

Subcuticular-Intracellular Hemibiotrophic and Intercellular Necrotrophic Development of Colletotrichum acutatum on Almond

ABSTRACT The early infection and colonization processes of Colletotrichum acutatum on leaves and petals of two almond cultivars with different susceptibility to anthracnose (i.e., cvs. Carmel and Nonpareil) were examined using digital image analysis of light micrographs and histological techniques. Inoculated tissue surfaces were evaluated at selected times after inoculation and incubation at 20 degrees C. Depth maps and line profiles of the digital image analysis allowed rapid depth quantification of fungal colonization in numerous tissue samples. The results showed that the early development of C. acutatum on petals was different from that on leaf tissue. On petals, conidia germinated more rapidly, germ tubes were longer, and fewer appressoria developed than on leaves. On both tissues, penetration by the pathogen occurred from appressoria and host colonization was first subcuticular and then intracellular. On petals, colonizing hyphae were first observed 24 h after inoculation and incubation at 20 degrees C, whereas on leaves they were seen 48 to 72 h after inoculation. Intercellular hyphae were formed before host cells became necrotic and macroscopic lesions developed on petals >/=48 h and on leaves >/=96 h after inoculation. Histological studies complemented data obtained by digital image analysis and showed that the fungus produced infection vesicles and broad hyphae below the cuticle and in epidermal cells. In both tissues, during the first 24 to 48 h after penetration fungal colonization was biotrophic based on the presence of healthy host cells adjacent to fungal hyphae. Later, during intercellular growth, the host-pathogen interaction became necrotrophic with collapsed host cells. Quantitative differences in appressorium formation and host colonization were found between the two almond cultivars studied. Thus, on the less susceptible cv. Nonpareil fewer appressoria developed and host colonization was reduced compared with that on cv. Carmel.     

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.     

Novel infection strategies of Colletotrichum acutatum on ripe blueberry fruit

The infection and colonization process of Colletotrichum acutatum on ripe blueberry fruit from two cultivars with different susceptibility to anthracnose were examined using light and confocal laser scanning microscopy. Ripe fruit from susceptible cv. Jersey and resistant cv. Elliott were drop-inoculated with a conidial suspension of C. acutatum, and epidermal peels were evaluated at selected times after inoculation and incubation. Results from pre-penetration studies demonstrated that there were significant differences in the rate of formation of melanized appressoria between the two cultivars, with the rate of formation being faster in the susceptible one. In both cultivars, penetration by the pathogen occurred via appressoria 48 h post-inoculation (hpi). However, in the susceptible cv. Jersey, C. acutatum then adopted an intracellular hemibiotrophic-like infection strategy, whereas in the resistant cv. Elliott subcuticular intramural-like infection occurred. In cv. Jersey by 108 hpi, intracellular growth of the pathogen led to the formation of numerous acervuli, with orange conidial masses. By 120 hpi, the conidial masses had coalesced covering the entire inoculated area. In cv. Elliott, acervuli were not seen until 144 hpi and contained few conidia. These results demonstrate for the first time the ability of C. acutatum to adopt a different infection and colonization strategy depending on the susceptibility of the host tissue being colonized.     

Colonization by two isolates of Plasmodiophora brassicae with differing pathogenicity on a clubroot-resistant cultivar of Chinese cabbage (Brassica rapa L. subsp. pekinensis)

A commercial clubroot-resistant F₁ cultivar of Chinese cabbage (Brassica rapa L. subsp. pekinensis), Kukai 70, is resistant to an isolate of populations of Plasmodiophora brassicae from Hagi (HG) city and is susceptible to another from Yamaguchi (YMG) city. The degree and frequency of primary and secondary cycle colonization by the isolates in the root hairs and root tissues of cv. Kukai 70 were compared. Seedlings of cv. Kukai 70 were grown in soils amended with inoculum of either HG or YMG and harvested 10 days after inoculation to observe the primary cycle (number of root-hair infections) and 20, 30, and 40 days after inoculation to observe of the secondary cycle (frequency of infected cells and degree of plasmodial development based on the number of nuclei in infected cells). Although more root hairs were infected in HG than in YMG, fewer cells in root tissues including the cortex and medullary rays were infected in HG than in YMG. In addition, YMG developed plasmodia with many nuclei and formed resting spores, whereas plasmodia remained immature with a small number of nuclei in HG and did not form resting spores even by 40 days after inoculation. These results suggest that suppression of plasmodial development during secondary colonization is associated with resistance mechanisms to HG in cv. Kukai 70. Starch did not accumulate (i.e., development of amyloplasts) in HG-infected cells. This may be involved in the suppression of secondary colonization of P. brassicae in the cultivar.     

Modelling the effects of wetness duration and fruit maturity on infection of apple fruits of Cox's Orange Pippin and two clones of Gala by Venturia inaequalis

The effects were investigated of fruit maturity and duration of wetness on infection of apple fruits by Venturia inaequalis , and subsequent scab development. Incubation rate (inverse of median incubation period) increased linearly with increasing temperature (5–20°C) on detached 5-week-old fruits of cv. Royal Gala. Fruits were highly susceptible in the early stages of development, but became increasingly resistant as they matured. Inoculation of attached 12-week-old and detached near-mature fruits did not result in any lesions, while inoculation of attached 4-, 5-, 7- and 9-week-old fruits resulted in various levels of infection. Fruits of cv. Mondial Gala were more susceptible than those of cv. Cox's Orange Pippin. On cv. Mondial Gala, a wet period of 9 h resulted in ≈ 90% infection of 4-week-old fruits, but only 9% infection of 9-week-old fruits. Numbers of scab lesions on an apple generally followed a Neyman type A rather than a Poisson distribution, indicating a certain degree of aggregation of lesions on a fruit. A two-parameter generalization of the Poisson model described the observed incidence–density relationship well. A longer duration of wetness was required to result in a similar level of scab infection on old fruits to that on young fruits. On cv. Mondial Gala, wet periods of 9 and 32 h were required for ≈ 90% incidence of fruit scab on 4- and 7-week-old fruits, respectively. A mathematical model was developed to relate the incidence of fruit scab to duration of wetness and fruit maturity. The potential use of these results in practical disease management is discussed.     

The use of a tomato cotyledon test to identify Corynebacterium michiganense pv. michiganense

When cotyledons of 4-day-old tomato seedlings (Lycopersicon esculentum cv. Sonato) were inoculated with Corynebacterium michiganense pv. michiganense and kept at 25°C with a 16 h light period per day, white raised blisters were produced which coalesced to give white, scurfy, cracked areas after 3 days. This symptom did not develop when Gram-positive coryneform bacteria isolated from, and near, tomato crops were used as inoculum. Pre-inoculation abrasion of cotyledons, an inoculum concentration of at least 10⁵ bacteria/ml, and high humidity after inoculation were important factors in the expression of this host reaction. The cotyledon test may be of value in laboratories where more detailed tests for C. michiganense pv. michiganense are not possible.     

Methods for assessing the susceptibility of potato tubers of different cultivars to rotting by Erwinia carotovora subspecies atroseptica and carotovora

The susceptibility of tubers of different potato cultivars to soft rot by Erwinia carotovora subspp. uroseptica and carotovora was assessed in 3 years by two methods. In one method, whole tubers inoculated at wounds with either bacterium were incubated under anaerobic conditions for 5 or o days at 15°C. In the other method, wounds made in tuber slices were allowed to heal or not, before inoculation with different concentrations of each bacterium and were then incubated under aerobic conditions for 3 days at 15°C. Most cultivars gave consistent reactions in repeated experiments using the same method, but there was some seasonal variation. A few cultivars were consistently susceptible (Klondyke and Manna) or resistant (Drayton) in both methods but others gave completely contrasting results (Record). In both methods and with all cultivars more rotting was caused by subsp. atroseptica than by subsp. carotovora because of the temperature of Incubation.     

Early development of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) in relation to temperature and leaf wetness.

In controlled environment experiments to study early development of light leaf spot, lesions developed with leaf wetness durations of 16 to 48 h after inoculation of oilseed rape with conidial suspensions of Pyrenopeziza brassicae at 12 or 18°C, but not with leaf wetness durations of 0 to 13h. The incubation period was 21 to 22 days at 12°C and 14 to 18 days at 18°C for leaf wetness durations of 16 to 48 h. The latent period was 21 to 23 days at 12°C and 18 to 19 days at 18°C, and the total number of lesions increased with increasing leaf wetness duration at both temperatures. In field experiments, light leaf spot always developed on oilseed rape with a leaf wetness duration of 48 h after inoculation in both 1990/1991 and 1991/1992, but the percentage leaf area affected was less on plants placed in an oilseed rape crop than on those placed in a glasshouse. Plants moved to an oilseed rape crop immediately after inoculation nearly always developed light leaf spot symptoms when they were inoculated between 19 October 1990 and 1 March 1991 or between 27 September 1991 and 14 February 1992, but plants inoculated between 31 August and 16 October 1990 or on 20 September 1991, when estimated leaf wetness duration was less than 16 h for several days after they were placed in crops, did not develop symptoms. The latent period of light leaf spot on plants transferred to the oilseed rape crop was 15 to 40 days, and there was an approximately linear relationship between 1 (latent period) and mean temperature during this period. The accumulated temperature during the latent period ranged from c. 150 to 250 day-degrees. The severity of lesions on these plants increased with increasing temperature from 5 to 15°C.     

The effect of temperature before or after inoculation with Phoma exigua var. foveata on the development of gangrene in potato tubers

Potato tubers of five cultivais were incubated at five temperatures between 2 and 20°C for 1 month and then inoculated with Phoma exigua var. foveata and stored at 5°C. Larger rots usually developed on tubers previously incubated at the higher temperatures but cultivar differences were greatest where tubers had been incubated at 2°C. In another experiment, rot size was related to the duration of incubation at 20°C before inoculation.
In a series of experiments, tubers were stored at different temperatures after inoculation; other factors (e.g. wound type or r.h.) were also investigated in some of these. Combining the results of all experiments a linear decrease of % wounds infected (logit transformation) with storage temperature was demonstrated. The size of rots was not simply related to temperature, but tended to increase over the range 2–10°C and then stabilized or declined at higher temperatures. Relative humidity treatments of 75% v. 95% did not consistently affect either the incidence of rotting or rot size. The other factors investigated did not usually interact significantly with temperature.
The relevance of the results for resistance testing is discussed and a simple equation relating disease incidence to inoculum levels, type of damage and temperature is proposed.     

Homothallism in Peronospora viciae f.sp. pisi and the effect of temperature on oospore production

Monoconidial cultures derived from seven P. viciae f.sp. pisi isolates, obtained from different countries, were able lo produce oospores, Apparently, these isolates were homothallic. Oospore production of one isolate was studied at 5, 10, 15 and 20°C in systematically colonized shoots, and in local lesions on leaflets, stem parts and pods of the pisum sativum cv. Kelvedon Wonder. The number of oospores produced per gram systemically colonized tissue increased with temperature. In lesions of leaflets and of stem parts, including tendrils, petioles and main stem, most oospores were produced at 20°C. At 10°C, a few oospores were found in stem parts but none in leaflet lesions. At 5° C, no oospores were formed at all. In pods, moe oospores were produced at 15 and 20°C than at 10°C, but the numbers of oospores was smaller than in the other plant parts. Oospores formed at lower temperatures were larger than those formed at higher temperatures. At 20°C, similar oospore densities were found in leaflet lesions of three cultivars widely differing in resistance to downy mildew.