Overwintering of Sphaerotheca mors-uvae on black currant and gooseberry

The ability of Sphaerotheca mors-uvae to perennate as cleistocarps, and as mycelium in buds was examined during the winters of 1965-6, 1966-7 and 1967-8 in relation to its two principal hosts, gooseberry and black currant. Cleistocarps on black currant leaves were examined from August 1965 to April 1966 and from July 1966 to March 1967. In 1965 cleistocarps were first observed on the leaves on 5 August; in 1966 on 11 July. These continued to develop through August and September and by October approximately 70% contained well-defined ascospores. The ascospore content remained generally at this level until February 1966 and November 1966; then the numbers of cleistocarps with ascospores fell and by April 1966 and March 1967 few such cleistocarps remained. From 21 March 1966 and 15 February 1967, but not otherwise, discharge of ascospores from the overwintered cleistocarps was readily obtained in laboratory tests. The viability and infectivity of the ascospores was demonstrated by allowing them to discharge on to leaf discs of black currant in the laboratory and also on to leaf discs and plants in the field. Sporulating colonies of S. mors-uvae developed within 8 days. Cleistocarps from shoots of black currant were examined from 4 August 1966 to 9 March 1967, and from 27 July 1967 to 1 January 1968. They developed in a similar manner to those on black currant leaves and by September in both 1966 and 1967 over 60% contained ascospores. This level was not maintained; the number of cleistocarps with ascospores fell gradually and by 8 December 1966 and 1 January 1968 few remained. Only in one laboratory test (21 November 1967) were ascospores discharged from a sample of these cleistocarps. Cleistocarps from shoots of gooseberry were examined from July 1966 to March 1967, and from August 1967 to January 1968. The pattern of ascospore development and subsequent decline in number of cleistocarps with ascospores was similar to that observed for black currant shoots. No discharge of ascospores could be demonstrated in laboratory tests. Evidence that S. mors-uvae perennates in buds of gooseberry was obtained by dissecting buds and by inducing buds on surface-sterilized shoots to burst under conditions which precluded chance infection. Field observations also suggested that bud infection occurred on gooseberry. Similar experiments failed to demonstrate the fungus in buds of black currant, and there was no indication of bud infection of this host in the field.     

Overwintering of Sphaerotheca humuli, the cause of hop powdery mildew

The ability of Sphaerotheca humuli to overwinter as cleistocarps in infected hop cones and leaves and in aerial buds on rootstocks was examined during the winters of 1970-1, 1971-2 and 1972-3. Periodical examination of cleistocarps, collected in October and overwintered in Terylene bags on the soil of a hop garden, consistently revealed two periods of maturation ending in November and in March, when over 50% contained eight, well-defined ascospores. In laboratory tests cleistocarps, kept either in the hop garden or dry at 4, 8 or 18^oC during the winter, could not be encouraged to dehisce earlier than April when naturally dehisced cleistocarps were first detected in the field. More ascospores were discharged from cleistocarps, and germination of ascospores in laboratory tests was greater, at 18 than at 4, 8 or 24^oC. Colonies of S. humuli arose on leaves of potted plants exposed to overwintered cleistocarps in the hop garden and were observed microscopically to originate from ascospores. However, a Burkard spore trap, operated amidst the cleistocarps in this garden in 1972 and 1973, failed to detect ascospores. Ascospores, discharged onto susceptible leaves in the laboratory, germinated but failed to produce colonies. It was demonstrated that S. humuli can perennate in aerial, dormant buds on hop rootstocks. Examination of buds in autumn revealed mycelium external to and between the bud scales. At budburst the mycelium was still present internally. Cleistocarps were occasionally associated with hibernating mycelium. Primarily infected shoots arose from plants bearing infected buds in conditions which precluded chance infection. Some evidence was obtained that conditions during the winter determine the success of survival in buds. The fungus appeared to be incapable of infecting a selection of weeds common to hop gardens and their vicinity.     

Resistance to powdery mildew in rose

Resistance and susceptibility of rose species and varieties to the powdery mildew fungus Sphaerotheca pannosa were tested on detached leaflets in Petri plates. Strains of the pathogen from greenhouse plants were maintained under aseptic conditions on rose shoots growing in agar medium in glass vials. Biological specialization in S. pannosa was confirmed; the strain on Rosa virginiana did not infect common rose varieties. Variation in susceptibility of commonly cultivated rose varieties to another strain of mildew was demonstrated, and resistance increased with age of leaf. Germination and prepenetration development of the fungus were not related to disease resistance. Fewer fungus haustoria developed normally in epidermal cells of resistant than of susceptible rose varieties. The relative importance of morphological barriers and internal resistance factors is discussed. Attempts failed to infect freshly isolated or long established callus tissue cultures of several rose varieties.     

Common Leafspot of Alfalfa: Ascospore Discharge and Plant Infection in the Field

Peak daily discharge of Pseudopeziza medicaginis ascospores in the field occurred after sunrise with attendant rises in temperature, wind, speed, and. dissipation of moisture. Protracted discharge over many days rose with initial development of robust, uncrowded apothecia and fell as these became spent and were joined by less productive, small, crowded ones. In the laboratory, detached field-infected leaves discharged ascospores at 5°C to 32.5°C and at relative humidities as low as 90% when fresh, and at 94% or higher when wilted or dried. Pathogen-free alfalfa plants placed overnight in a diseased affalfa plot were inoculated but uninfeeted more frequently in July than they were inoculated and infected. Artificially-inoculated plants placed at the same time in a nearby sugar beet plot became infected every overnight. From early September through early October artificially inoculated plants placed in a plot of sugar beets or a plot of nearly disease-free alfalfa for 24 h periods, became infected in 18 and 19 of 29 such periods. Light rain, fog and dew were frequent during these periods and early morning temperatures fell between - 3 °C and 5 °C- in one-half of the overnights. Dormant ascospores on foliage remained fully infective in the greenhouse for at least 10 days.     

Podosphaera pannosa (syn. Sphaerotheca pannosa) on Rosa and Prunus spp.: Characterization of Pathotypes by Differential Plant Reactions and ITS Sequences

The powdery mildew fungus Podosphaera pannosa (Wallr.: Fr.) de Bary (syn. Sphaerotheca pannosa) is a major problem on roses worldwide. Twenty‐six monoconidial isolates of Podosphaera collected on roses and Prunus spp. in Belgium, Germany, France, Denmark, Israel and The Netherlands were characterized on the basis of differential reactions on in vitro rose genotypes and Prunus avium L. and by DNA sequence analysis of the rDNA ITS (internal transcribed spacer) region. Twenty‐four isolates were determined as P. pannosa. Amongst these, different groups could be distinguished. A first group of 18 isolates was highly virulent on rose and avirulent or very weakly virulent on P. avium. A second group of four isolates was highly virulent on both rose and P. avium. Analysis of the ITS sequence could discriminate these two groups of P. pannosa strains by a one base pair difference. Finally, two isolates of powdery mildew collected on Prunus sp. could be classified as P. pannosa based on their ITS sequence, which was identical to the ITS sequence of the isolates only highly virulent on roses. However, these two isolates were not able to infect roses. These results indicate that different strains of P. pannosa exist with varying host specificity. We demonstrated by ITS sequencing and plant reactions that the host range of P. pannosa comprises roses and Prunus spp.     

The role of the apple spray programme in the protection of fruit buds from powdery mildew

Comparisons of programmes, which differed in the dates on which high-volume sprays of dinocap (0.019 %) were omitted, indicated that applications in the period from green cluster to early fruitlet protected the bourse buds of apple cv. Cox's Orange Pippin from infection, and so controlled the number of mildewed blossom trusses in the following year. This role of the sprays in preventing primary mildew declined after the early fruitlet stage. The existence of two phases of the disease was confirmed, and although dinocap at late blossom and early fruitlet contributed to the control of preliminary infections of secondary mildew on vegetative shoots, the latter phase did not become severe until after the peak invasion of bourse buds. Fewer than 20% of the fruit buds which produced primary-mildewed blossom in 1968 were succeeded by healthy fruit buds in the next year; this poor recovery was not improved by dinocap applied during the flowering period in 1968.
Fruit-set and crop of James Grieve and Cox's Orange Pippin were reduced in 1968 by dinocap applied at green cluster and pink bud, or at full blossom and petal-fall, but no deleterious effects were produced by equivalent sprays in 1969. The need for mildew control during flowering is discussed in relation to phytotoxic sensitivity.     

Population Structure and Management of Podosphaera pannosa Associated with Peach Powdery Mildew in Oman

In 2004, severe powdery mildew infection on peach occurred in Al‐Jabal Al‐Akdhar, Oman, and resulted in substantial yield losses to growers. This study was conducted to investigate occurrence, causal agents, genetic diversity and efficacy of azoxystrobin in management of this disease. Powdery mildew was observed on all farms and peach trees in Al‐Jabal Al‐Akdhar. Disease symptoms were first observed on shoots in April, followed by appearance on fruits. Disease severity reached its peak between May and June. Morphological and molecular identification of 22 powdery mildew isolates indicated that all belong to Podosphaera pannosa. Podosphaera pannosa reproduced the same symptoms upon inoculation on peach leaves. Amplified fragment length polymorphisms analysis of 35 isolates of P. pannosa from five different villages using four primer pair combinations produced 688 polymorphic loci and 35 different genotypes. Populations of P. pannosa were found to have low levels of gene diversity (H = 0.1858), which suggests that P. pannosa has been recently introduced into Al‐Jabal Al‐Akdhar. Analysis of molecular variance showed low levels of genetic differentiation among populations from the different villages, implying the introduction of P. pannosa into the different villages via common sources as well as frequent movement of pathogen inoculum among the different villages. Evaluating the efficacy of azoxystrobin showed that azoxystrobin is as efficacious as thiophanate‐methyl in managing the disease, with sulphur being the least efficacious. The study is the first to report the presence of P. pannosa in Oman. Also reported are its genetic diversity and its management under commercial conditions.     

Infection periods in relation to the natural development of hop downy mildew (Pseudoperonospora humuli)

The relationships between temperature and surface wetness and subsequent infection of hop tissues by P. humuli were examined on potted plants and detached leaves kept in temperature-controlled growth rooms. Periods of wetness which would just allow leaf infection ranged from 1 1/2 h at 30d? to 24 h at 5d?. The corresponding ranges for shoots were: light infection, 3 h at 19–23d? to 6 h at 8–10d?; severe infection, 4 h at 19–23d? to 8 h at 12–13d?. These data were used to relate the development of downy mildew in an unsprayed hop garden during 1967 and 1968 to periods with temperature/surface wetness suitable for minimum (minor infection periods) and severe infection (major infection periods). In 1967 a sudden outbreak of infected basal shoots (spikes) was related to an isolated major infection period. By contrast, early in 1968, major shoot infection periods did not arise and spikes appeared gradually in response to a succession of minor infection periods. More spikes were formed in 196 than in 1967; this was not related to the incidence of infection periods but probably reflected the relatively higher concentrations of airborne sporangia early in 1968. In both years outbreaks of leaf and lateral shoot infection could be traced to major infection periods caused by rain; sudden disease increases again originated from isolated infection periods. There was a close similarity between the incubation period for each principal disease outbreak and that expected from growth-room experiments. Major infection periods occurred more frequently at the end of June 1968, resulting in a higher final concentration of diseased tissue than in 1967. Predicted major infection periods failed to induce large disease increases when dew alone provided wetness or when no airborne sporangia could be detected.     

The inhibitory effects of rose powdery mildew infection on the oviposition behaviour and performance of beet armyworms

In this study, we investigated whether the oviposition behaviour and performance of the beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), on the rose cultivar Rosa chinensis Jacq. (Rosaceae) were affected when the plants were infected by rose powdery mildew, Podosphaera pannosa (Wallr.: Fr.) de Bary (Erysiphales). The bioassays revealed that the moths significantly avoided ovipositing on mildew‐infected rose leaves when compared to healthy leaves. Pupal weights, emergence rates, and fecundity decreased when the caterpillars were fed mildewed rose leaves. Further laboratory bioassays aimed to elucidate the effects of two volatile headspace extracts (separately collected from healthy and mildewed rose plants) on the oviposition behaviour and performance of the moths. The moths clearly preferred to oviposit on healthy rose leaves that were not sprayed with additional volatiles rather than on healthy leaves sprayed with the volatile extracts from mildewed plants. The mean number of eggs laid on the former leaves was more than six times higher than that laid on the latter leaves. Olfactory bioassays demonstrated that ovipositing moths were significantly more attracted to volatiles emitted by healthy rose leaves than to those emitted by mildew‐infected leaves. Similar results were obtained when comparisons were made between the volatile extracts collected from healthy and mildewed rose plants. Thus, volatiles from mildew‐infected roses have a strong inhibitory effect against the moths. These results indicated that rose volatiles play a role in the oviposition behaviour of the moths, and that the volatiles induced by powdery mildew might be used for insect control.     

Environmental Factors involved in the Regulation of Sprouting of Basal Buds in Rose Plants

The plant top is the main factor inhibiting the formation ofjuvenile-like shoots from the basal part of the rose plant.In plants with the whole top removed a cooling period of oneweek at 4 °C had a promotional effect on the outgrowth ofbasal buds. Six days of darkness partially inhibited sproutingof these buds and 9 d suppressed sprouting completely. On theother hand. 3 d in light following plant top removal were sufficientto cause sprouting. Buds on the upper parts of lateral shoots were not affectedby the dark treatment. Key words: Basal buds, Rejuvenation, Roses     

Powdery mildew resistance in roses: QTL mapping in different environments using selective genotyping

Podosphaera pannosa, the causal agent of rose powdery mildew, hampers the production of cut roses throughout the world. A major tool to control this disease is the use of resistant plant material. Single resistance genes, like Rpp1, may be overcome within a few years by high risk pathogens like powdery mildews. Durable resistance could be achieved using quantitative resistances. Here we describe mapping of QTLs for resistance to P. pannosa in six different environments (artificial and natural infections in the greenhouse over 3 years and natural infections in the field over 2 years). AFLPs, RGAs and other marker types were used to construct an integrated linkage map for the diploid population 97/7 containing 233 markers. In a selective genotyping procedure, marker segregation was analysed for 170 of the up to 270 phenotyped individuals. We identified seven linkage groups with an average length of 60 cM, corresponding to seven rose chromosomes in the haploid set. Using an LOD significance threshold of 3.9 we detected a total of 28 QTLs for the nine powdery mildew disease scores under analysis. Using the data from artificial inoculations with powdery mildew race 9, three resistance QTLs explaining about 84% of the variability were mapped. Twelve and 15 QTLs were detected for resistance to naturally occurring infections in the greenhouse and in the field, respectively, over several years.     

Host-Parasite Relationship in a Susceptible and a Resistant Rose Cultivar Inoculated with Sphaerotheca pannosa.

Young leaves of two rose cultivars, one susceptible (rose Cardinal) and the other moderately resistant (rose Q. Elizabeth) to mildew infection, were either inoculated with Sphaerotheca pannosa or wounded with glass or metal needles. At different times after these treatments the presence of callose, lignin and phenolics in the infected or wounded cells was revealed by histochemical reactions. After either treatment no difference was found between the cultivars in the amount or earliness of deposition of callose or lignin, while phenolics were present in far larger amounts in the resistant cultivar. It is therefore suggested that one of the factors of rose resistance to powdery mildew might be the capacity to synthesize large amounts of phenolic compounds, also in view of the fact that the vacuoles of healthy epidermal cells are strongly electron opaque in rose Q. Elizabeth, while they are much more transparent in rose Cardinal.     

The timing of sprays for the protection of terminal buds on apple shoots from powdery mildew

Post-blossom sprays of fungicides, repeated at 10-day intervals until leader (syn. extension) shoots had stopped producing new leaves, provided the best protection of terminal buds against Podosphaera leucotricha on the apple cv. Lane's Prince Albert. Spraying was most effective in early summer, although many of these buds were not invaded until later, when the rate of shoot growth declined; applications from July to September did not compensate for the enhanced infection which followed interruptions of the post-blossom programme between late May and early July. This early period was critical because most leaf infections occurred then, and because this phase of the epidemic on foliage determined the eventual intensity of mildew on terminal leaves, and hence the inoculum available for infecting terminal buds. Also, many lateral shoots ceased growth early and their apices were directly protected by sprays applied in June. Applications after early June were too late to protect newly formed fruit buds on spur branches.     

Frost Injury as a Possible Inciting Factor in Bud and Shoot Necroses of Fraxinus excelsior L.

Large numbers of European ash have died in Poland in all age classes during the last ten years. The characteristic symptom occurring on shoots of planted and self‐sown seedlings was bark necroses starting from the shoot apex, necrotic buds, or leaf and twig scars. The results showed that in the bud tissue of cold acclimated European ash extracellular and intracellular ice formation occurred at approximately ?9 and ?32°C, respectively. In deacclimated plants in spring water supercooling is limited by the heterogenous ice nucleation temperature and consequently the cold tolerance is ?9 to ?4°C for bud tissues and ?13 to ?9°C for shoots. Isolations of fungi were performed from dead buds and from necroses occurring on the main stem. Alternaria alternata, Fusarium lateritium and Phomopsis scobina were among the fungi occurring in both these organs at frequencies of more than 7%. Cylindrocarpon heteronemum, Diplodia mutila and Tubercularia vulgaris from necroses were only isolated in frequencies; 3.3, 1.2 and 5.4%, respectively. It seems likely that freezing injury is the inciting factor, which combined with fungal colonization manifests itself as fatal damage to European ash buds and shoots.     

A thermal time model for predicting time to aerial shoot elongation in Variegated Solomon's Seal

Aerial shoot development in Variegated Solomon's Seal (Polygonatum odoratum‘Variegatum’) was studied under warm (mean 18°C) conditions after dormant rhizomes had been stored at a range of temperatures. After chilling at 0.8–5.5°C for 21–77 days, all rhizomes produced elongated aerial shoots, with mean lengths from 33 cm to 44 cm. Exposure of rhizomes to 15°C or 20°C for 21–77 days resulted in 17% to 50% of buds emerging as shoots, but these either aborted or failed to extend beyond a rosette. The earliest aerial shoot elongation was observed after 7–13 days at 18°C in rhizomes that had been chilled at 0.8–2°C treatments for 59–72 days. The base, optimum and maximum temperatures during pre‐planting storage for subsequent aerial shoot elongation were derived respectively as ?1.5°C, 1.9°C and 15.8°C. A thermal time of storage was calculated from these cardinal temperatures and the rate of progress to normal aerial shoot clongation was shown to increase linearly with increasing thermal time to c. 150°Cd. The thermal time procedure for predicting time to aerial shoot elongation constructed from growth room/chamber data was validated using rhizomes that had been exposed to varying temperatures in shadehouse conditions during the winter in Taiwan.     

In vitro regeneration via caulogenesis and brassin-induced shoot conversion of dormant buds from plumular explants of peanut (Arachis hypogaea L. cv `Okrun')

Shoot buds were induced from plumular explants of peanut (Arachis hypogaea L., cv `Okrun') preconditioned on medium containing 2,4-dichlorophenoxyacetic acid and kinetin and then transferred to regeneration medium containing benzylaminopurine and β-naphthoxyacetic acid. Buds differentiated 25 days following transfer to regeneration medium. Each explant produced 30 to 40 buds, but only 4 shoots. The remaining buds were dormant and did not produce shoots when maintained on regeneration medium. Shoots were regenerated continuously, however, when explants were subsequently transferred to shoot conversion medium containing 1 μM brassin, benzylaminopurine and β-naphthoxyacetic acid, respectively. Approximately 5 shoots were harvested every 30 days after transfer to shoot conversion medium for up to 7 months. No further shoot production was observed from explants maintained on regeneration medium without brassin. Regenerated shoots could be rooted and produced viable seeds. This procedure provides an efficient and reliable system for regeneration and transformation studies using cv `Okrun'. Received: 9 April 1997 / Revision received: 27 August 1997 / Accepted: 20 September 1997     

Observations on the life cycle and establishment ofCochylis atricapitana (Lep: Cochylidae), a moth used for biological control ofSenecio jacobaea in Australia

Larvae of the mothCochylis atricapitana (Stephens) are monophagous leaf, crown, stem or bud borers of ragwort,Senecio jacobaea L. (Asteraceae). In the present investigation, aspects of the life cycle ofC. atricapitana were determined. Moths ofC. atricapitana lay an average of 158 eggs/female with as many as 355 eggs being laid by a single female. The majority of eggs are laid individually along the primary and secondary veins on the underside of ragwort leaves. Egg incubation ranges from 4.2 days at 30°C to 14.4 days at 15°C. At a constant 23°C under a 16 hour photoperiod,C. atricapitana takes approximately 40 days to complete a generation. Caterpillars make their way to young, actively growing ragwort shoots or buds, and begin mining into the plant tissue, boring into the leaf, crown, stem or bud.C. atricapitana has five larval instars and enters diapause as a final instar larva. In southern Victoria, moths ofC. atricapitana fly from late September through to the beginning of February. Adults emerge after overwintering towards the end of spring or beginning of summer.C. atricapitana has established at two sites while larvae, or signs of damage have been observed at approximately 52% of release sites.      

Characterization of Reactions to Powdery Mildew (Podosphaera pannosa) in Resistant and Susceptible Rose Genotypes

Fungal development of powdery mildew Podosphaera pannosa (Wallr.: Fr.) de Bary on rose leaves depends on constitutive or induced resistance mechanisms present in attacked rose genotypes. The relationship between fungal development and plant resistance was investigated microscopically on young greenhouse leaves of four rose genotypes with different levels of resistance: Rosa wichuraiana, R. laevigata anemoides and R. hybrida cultivars ‘Excelsa’ and ‘Gomery’. Induced plant reactions, hydrogen peroxide production and cross sections through infected leaves were examined. The variation in development of the fungus on these rose genotypes depended on the relative presence of normal haustoria, abnormal haustoria, induced cell reactions, papilla formation or physical barriers. Formation of papillae could arrest up to one third of the successful penetrations. Papillae formation was often succeeded by total cell reaction. Abnormal haustoria were detected as rudimentary haustoria, haustoria with abnormal shape or haustoria without extra haustorial matrix. Post‐haustorial cell reactions, with and without cell collapse, were detected. In non‐collapsed cells, appositions were directed to both cell wall and haustorium. This was followed by accumulation of non‐identified, probably antifungal compounds. Both single and multicell reactions occurred. Hydrogen peroxide was detected during papilla formation and induced cell reactions.     

Using aerated compost tea in comparison with a chemical pesticide for controlling rose powdery mildew

Rose powdery mildew (Sphaerotheca pannosa var. rosae) is one of the most common foliar diseases of roses worldwide. Application of chemical products on the plant or in the soil kills a range of the beneficial micro-organisms thereby disturbing ecosystem. Compost tea helps to restore and increase the populations of those beneficial micro-organisms. The objective of the present study was to evaluate the comparison of biopesticide (compost tea) and a chemical pesticide. The experiment was performed in three treatments, which were compost tea, fungicide (Topaz) and no treatment in three replications. After foliar applications of biopesticide and fungicide, the control percentage was estimated based on the number of infected flowers with powdery mildew. The results indicated that there was a significant difference between these treatments on rose in controlling powdery mildew (F?=?23.25, p?=?0.0015, df?=?2), at a probability level of 1% (p???0.01). So, that control percentage of compost tea treatment was the most.     

Forecasting the airborne spread of Mycosphaerella fijiensis, a cause of black Sigatoka disease on banana: estimations of numbers of perithecia and ascospores

Banana leaves showing different levels of black Sigatoka disease were collected from an unsprayed plantation in Costa Rica during two separate periods representing the wet to dry season transition (October 1993 – February 1994) and the dry to wet season transition (April – September 1995). Laboratory studies were used to investigate the relationship between the release of Mycosphaerella fijiensis ascospores and the amount of inoculum on banana plants showing different levels of infection, as assessed by leaf necrotic area. The number of perithecia present in the necrotic area was used as an indication of potential ascospore loads and was investigated as a series of regression equations. A series of rewetting and incubation regimes was used to investigate spore release under field conditions (21°C and 100% relative humidity in the early morning and 28°C, 60% relative humidity on days when it rained in mid-afternoon). Results suggest that rainfall, combined with a high temperature, may lead to peaks of ascospore release but without necessarily increasing overall numbers released over periods of up to 4 days and that a high level of spore release was less sensitive to changes in temperature once it had been initiated. The exact role of temperature in spore release is still unclear, however, as leaf samples kept at atypically low temperatures also released non-germinating ascospores. An average of 4.5 ascospores was released per perithecium. This does not resolve ambiguities in the literature regarding the number of ascospores present in each perithecium. A linear model relating the average ascospore numbers and necrotic area, using quick estimates of the amounts of necrotic area on the leaves of a random sample of plants across a plantation, is proposed, to give an indication of the relative amount of airborne inoculum potentially available between different plantations.