Downy Mildew Development Research Articles

Methods for screening maize against downy mildew Peronosclerospora sorghi

AbstractMethods of inoculum delivery and timing of test‐row planting were assessed for efficacy in promoting development of downy mildew (Peronosclerospora sorghi) in susceptible maize, and for cost of implementation in terms of personnel and labour. Direct inoculation of pre‐germinated seed of spreader rows resulted in consistent and high incidence of downy mildew infected plants and required substantially less labour and inoculum than the spray inoculation of spreader rows. The optimum time to plant test rows was around 15 days after inoculation of spreader rows. Eight breeding lines were screened for resistance to downy mildew using the revised inoculation method. After one cycle of screening with the new method, resistance levels improved by a range of 0–44% over the previous cycle.

Comparative Study of Microclimate and Downy Mildew Development in Subsurface Drip- and Furrow-Irrigated Lettuce Fields in California

Microclimates and downy mildew (caused by Bremia lactucae) disease progress were monitored in neighboring lettuce fields with subsurface drip or furrow irrigation during five trials in 1992 to 1993. The trials included a total of ten irrigation events during which soil moisture, soil temperature, canopy air temperature and humidity, leaf wetness duration, wind speed, and solar radiation were recorded. Disease intensity was assessed at intervals, beginning at thinning of the crop and ending just before harvest. Wilcoxon's Signed Rank Tests were computed to compare microclimates between drip- and furrow-irrigated fields, separately for days before or after irrigation. There were no significant differences in microclimate between the two irrigation methods before irrigation. Within 3 days after irrigation, there were significantly longer overall leaf wetness periods (P ≤ 0.0025) and a trend toward higher daytime humidity (P ≤ 0.1254) and longer morning leaf wetness periods (P ≤ 0.0863) in fields with furrow irrigation. Air temperature and nighttime humidity were not consistently different between the two irrigation methods. Downy mildew developed in four of the five trials, and disease intensity was always lower under drip irrigation than under furrow irrigation. The magnitude of the differences in disease was small, however. It appears that, on most days in coastal California, mesoclimatic variations outweigh microclimatic modifications that could potentially influence disease development

Weather Variables Associated with Infection of Lettuce by Downy Mildew (Bremia lactucae) in Coastal California

Weather conditions and downy mildew (Bremia lactucae) development were monitored in 13 commercial lettuce fields in the central coast production areas of California during 1991 and 1992. Days on which infection likely had occurred were identified based on observations of the appearance of new lesions combined with quantitative information about the length of the latent period of the disease. Univariate (Kolmogorov-Smirnov tests) and multivariate (stepwise discriminant analysis) procedures were applied to statistically differentiate infection days from days on which infection had not occurred and to characterize infection days in terms RACT of weather [...]

Validation of 'Downcast' in the prediction of sporulation-infection periods of Peronospora destructor in the Lockyer Valley.

The disease forecasting system 'Downcast' was developed in Canada to indicate sporulation-infection periods of Peronospora destructor (Berk.) Caspary in onions. During 1989, observations were made of downy mildew development in trap plants and of disease progress in an unsprayed plot of onions in the Lockyer Valley, Queensland, and related back to predicted sporulation-infection periods using the forecaster. Over 113 days, there were 20 days when sporulation and infection were predicted, and of these 14 occurred during 4 weeks in July and August. Of 16 groups of trap plants exposed in the field, each for 7 days, 7 were subjected to predicted sporulation-infection periods. Two predicted sporulation-infection periods early in the season did not result in infection of trap plants or of plants in the field plot, probably due to absence of inoculum. Subsequently, groups of trap plants exposed in the field during predicted sporulation-infection periods became infected while those exposed when weather was unsuitable remained healthy. 'Downcast' shows promise as a technique for more effective management of disease caused by Peronospora destructor.

PLASMO: a simulation model for control of Plasmopara viticola on grapevine1

The problem of protecting grapevine against diseases is an old one, but in the last few years new techniques have been developed to reduce cost to the farmer and damage to the ecosystem. These are based on mathematical models describing the state of the plant‐parasite environment system. A model for forecasting development of grapevine downy mildew (Plasmopara viticola) is presented. The input variables are temperature, rainfall and leaf wetness (determining infection by sporangia), and RH and temperature (for incubation period). The model also takes into account the limited survival of spores. The output is expressed as %, disease progress. Field validation tests, performed in 1990, 1991 and 1992 in several vineyards in Toscana (central Italy) showed a good correlation between observed and simulated infections. The model allowed the number of treatments to be reduced without any increase in downy mildew damage. It could in future be integrated with grapevine growth and development simulation models in an expert system to determine infected tissue area and thus the economic damage threshold.

PLASMO: a computer program for grapevine downy mildew development forecasting

A software implementation of PLASMO (Plasmopara Simulation Model) model for downy mildew ( Plasmopara viticola Berl. et De Toni) of grapevine ( Vitis vinifera L.) development forecasting is presented in this paper. The computer program has been developed to facilitate validation and further improvements and to allow direct model use in vineyard management. The model simulates the development of downy mildew on the basis of climatic conditions. The inputs to the model are: temperature and leaf wetness for the inoculation phase, temperature and relative humidity for the incubation phase and the type of treatment applied. Numeric or graphic outputs are available. The program provides useful information for identifying fungicide application times on the basis of the actual downy mildew development. Field validation tests performed in untreated rows show a good correlation between field observed and model simulated infections. Moreover the model identifies only three treatment distribution periods, while with the traditional method six treatments were applied.

Phosphonic acid controls downy mildew (Peronospora parasitica) in cauliflower curds

Postharvest development of downy mildew (Peronospora parasitica) symptoms in export cauliflower curds has resulted in economic losses to growers in south-western Australia. Neutralised phosphonic acid sprays applied onto cauliflowers in the field within 3 weeks of harvest reduced this disease. Two applications of 2.4 kg a.i./ha, 21 and 7 days before harvest, reduced the incidence of curds affected by downy mildew from 92% in untreated plots to 8%. The maximum phosphonate residue in curds at harvest was 12 �g/g, while control curds contained no detectable phosphonate. No effects of phosphonic acid on crop appearance or maturity were observed, and while a small (8%) reduction in curd weight was significant (P = 0.039), there was no yield trend with increasing phosphonic acid rate.

Effect of temperature and light on sporangial germination and zoospore infectivity inSclerospora graminicolaon pearl millet

The effects of temp. on germination of S. graminicola sporangia (zoospore release), zoospore viability and germ tube growth, and of light and temp. on zoospore infectivity and downy mildew development in pearl millet (Pennisetum glaucum [P. americanum]) were studied. Germination of sporangia was highest at c. 30 deg C and little germination occurred after 2 h 40 min. Germ tubes grew at 15-35 deg . Zoospores retained high infectivity ( more than or equal to 70% systemic infection ) for 4 h at 30 deg C, and for a longer time at lower temp. Zoospore infectivity was not affected by light. The shortest incubation period (3 d) and the highest infectivity (66%) were obtained at 30 deg . Although both roots and shoots of young seedlings were susceptible, incidence of downy mildew was highest when both were inoculated

Effect of Aliette on Bremia lactucae on lettuce

Aliette is a systemic fungicide effective against diseases caused by lower fungi but there is some question whether it acts directly on pathogens or indirectly via host defence. Downy mildew development on lettuce cotyledons was markedly decreased by application of 100–500 μg Aliette, active ingredient (a.i.), ml −1 to roots. Sporangial germination on glass and cotyledons was prevented by direct application of 10 and 25 μg Aliette ml −1 respectively. Germ-tube growth and penetration into cotyledons were markedly decreased when 25 μg Aliette ml −1 was applied directly 2 h after sporangial inoculation. The size of secondary vesicles and growth of intercellular hyphae in cotyledons were markedly affected when 250 μg Aliette ml −1 was applied 4 h after sporangial inoculation. Hyphal growth was seen to be affected 24 h later; a low incidence of necrosis of host cells was observed after a further 24 h. The findings gave reason to believe that Aliette acted directly on the pathogen in this system.

A comparison of two pathosystems: Downy mildew on Spinacia oleracea and on Chenopodium album

The development of downy mildew (Peronospora farinosa) on a cultivated spinach crop (Spinacia oleracea) was compared to that on lambsquarters (Chenopodium album), a naturally occurring weed. Experiments were carried out on small spinach plots, in which epidemics could develop from point sources (PS) or area sources (AS).

Three-dimensional representation of downy mildew development in a spinach crop

The development of downy mildew on spinach (Peronospora farinosa f. sp.spinaciae) was stratified according to leaf layers and represented in three-dimensional computer graphics, in which percentage diseased leaf area was plotted against time for each leaf layer. Distinction was made between a point source and an area source. Inoculations were made at three growth stages of the crop. More information could be gathered on the course of disease development on the upper leaf layers, for the first true leaf pair sporulates longer and more intensively, masking the disease development on the other leaf layers in this way. De ontwikkeling van valse meeldauw op spinazie (Peronospora farinosa f. sp.spinaciae) werd geanalyseerd aan de hand van drie-dimensionale grafische voorstellingen, waarbij percentage ziek bladoppervlak per bladetage werd uitgezet tegen tijd. Onderscheid werd gemaakt tussen de ontwikkeling vanuit een puntbron en vanuit een oppervlaktebron, bij inoculatie in drie groeistadia van het gewas. Het bleek dat hierbij meer informatie kon worden verkregen over het verloop van de ziekte-ontwikkeling op de bovenste bladetages, omdat het eerste echte bladpaar langduriger en heviger sporuleert, en zo de ziekte-ontwikkeling op de overige bladetages maskeert.

Downy mildew of Cucurbits (Pseudoperonospora Cubensis): the Fungus and its hosts, distribution, epidemiology and control

Taxonomy of the genusPseudoperonospora, morphology ofPseudoperonospora cubensis (Berk, et Curt.) Rostow. and occurrence of its oospores, are described briefly. A list is presented of over 40 cucurbitaceous host species, representing about 20 genera, on whichP. cubensis has been recorded. Two or more races exist in Japan and the United States, but not in Europe or the Middle East. The distribution ofP. cubensis is widest on all continents on cucumbers (70 countries) and muskmelon (50 countries); onCucurbita and watermelons it extends to about 40 and 25 countries, respectively. P. cubensis may overwinter as oospores, though this seems rare, and on wild hosts or crops grown in the open or under cover. Airborne sporangia may also reach cooler countries from regions with mild winters. Apart from the leaf wetness essential for infection, the factors determining disease progress are: rate of foliage growth and physiological age of the host; amount of primary inoculum available, light, and the rate at which lesions necrotize. The interaction of these factors is described for early, mid-season, and late crops. Losses caused byP. cubensis depend on the growth stage at which the crop is attacked, and on the rate of foliage and pathogen development. Breeding has produced downy mildew resistant lines of cucumbers, used chiefly in the United States, and some resistant lines of melons and watermelons. The most important agricultural practices used to restrict downy mildew development are proper irrigation management and avoidance of sowing in proximity to infected crops. Success of control by protectant chemicals depends largely on proper timing of applications. Proximity of inoculum sources, hours of leaf wetness, age of crop, and irrigation practices are the principal factors that determine when to begin treatments. These factors and rate of leaf formation determine the frequency of applications. Application of systemic fungicides is much easier to time correctly.

Relationship between mineral nutrition of muskmelon and development of downy mildew caused byPseudoperonospora cubensis

Sand culture technique was used to study the relationship between mineral nutrition of muskmelon and development of downy mildew. Different levels of nutritioni.e. normal, one fifth of normal, lack of all nutrients, and six solutions of unbalanced nutritioni.e. high nitrogen, low nitrogen, high phosphorus, low phosphorus, high potash and low potash were used. Development of downy mildew was greater on the plants grown in one fifth of normal nutrition. The disease development was less on the plants grown in high phosphorus, low potash and high nitrogen nutrition solutions. Nutrition of the host also affected the sporulability of downy mildew fungus. However, there was no correlation between disease intensity and sporulation.

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

SUMMARYThe 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 30d̀ to 24 h at 5d̀. The corresponding ranges for shoots were: light infection, 3 h at 19–23d̀ to 6 h at 8–10d̀; severe infection, 4 h at 19–23d̀ to 8 h at 12–13d̀. 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.