Downy Mildew Epidemics Research Articles

Downy mildew (Plasmopara viticola) epidemics on grapevine under climate change

AbstractAs climate is a key agro‐ecosystem driving force, climate change could have a severe impact on agriculture. Many assessments have been carried out to date on the possible effects of climate change (temperature, precipitation and carbon dioxide concentration changes) on plant physiology. At present however, likely effects on plant pathogens have not been investigated deeply. The aim of this work was to simulate future scenarios of downy mildew (Plasmopara viticola) epidemics on grape under climate change, by combining a disease model to output from two general circulation models (GCMs). Model runs corresponding to the SRES‐A2 emissions scenario, characterized by high projections of both population and greenhouse gas emissions from present to 2100, were chosen in order to investigate impacts of worst‐case scenarios, among those currently available from IPCC. Three future decades were simulated (2030, 2050, 2080), using as baseline historical series of meteorological data collected from 1955 to 2001 in Acqui Terme, an important grape‐growing area in the north‐west of Italy. Both GCMs predicted increase of temperature and decrease of precipitation in this region. The simulations obtained by combining the disease model to the two GCM outputs predicted an increase of the disease pressure in each decade: more severe epidemics were a direct consequence of more favourable temperature conditions during the months of May and June. These negative effects of increasing temperatures more than counterbalanced the effects of precipitation reductions, which alone would have diminished disease pressure. Results suggested that, as adaptation response to future climate change, more attention would have to be paid in the management of early downy mildew infections; two more fungicide sprays were necessary under the most negative climate scenario, compared with present management regimes. At the same time, increased knowledge on the effects of climate change on host–pathogen interactions will be necessary to improve current predictions.

Importance of secondary inoculum of Plasmopara viticola to epidemics of grapevine downy mildew

To quantify the magnitude and the spatial spread of grapevine downy mildew secondary sporangia, 4685 Plasmopara viticola single lesion samples were collected from 18 plots spread across central Europe. Disease symptoms were collected on two to 22 sampling dates per plot between 2000 and 2002. Four multiallelic microsatellite markers were used for genotypic identification of pathogen samples. Genetic analysis showed more than 2300 site‐specific P. viticola genotypes, indicating that populations are genetically rich demographic units. Approximately 70% of the genotypes were sampled once and 14% were sampled twice throughout the various epidemics. In the 18 populations only seven genotypes (0.3%) were identified more than 50 times. Three genotypes particularly successful in causing disease through secondary cycles showed mainly a clustered distribution. The distance of sporangial migration per secondary cycle was less than 20 m and their plot colonization rate was calculated at around 1–2 m2 day−1. Downy mildew epidemics of grapevine are therefore the result of the interaction of a multitude of genotypes, each causing limited (or a few) lesions, and of a dominant genotype able to spread stepwise at plot‐scale. These findings contrast with current theories about grapevine downy mildew epidemiology, which postulate that there is massive vineyard colonization by one genotype and long‐distance migration of sporangia.

Valuable offsprings of Seyve-Villard 12375

Powdery mildew, Phylloxera and downy mildew epidemics devastated large areas of vineyards in Hungary in the 1800s. To establish new plantages resistant so called "direct producers" were imported from America (direct means "without grafting"). Direct producers have secure yield but their quality is inferior. After World War II, an organized breeding programme was started to replace them and parallel to this the law prohibited their production. As a resistance gene source, the E.2 clone (S. V. 12375 E.2) of Seyve-Villard 12375, Franco-American hybrid selected in Eger was used by Hungarian breeders. Several valuable hybrid families have originated from crosses of this variety hybrid.
 In the Research Institute for Viticulture and Enology at Kecskemét, two valuable table grape hybrids were registered under the names of R.65=Esther and R.78=Fanny coming from 2 hybrid families of the parent S. V. 12375 E.2. Both varieties may be cultivated "environmentally".

Efeito do míldio na concentração de sólidos solúveis totais em frutos do meloeiro

O míldio causado por Pseudoperonospora cubensis se constitui em uma das mais importantes doenças do meloeiro (Cucumis melo) no Nordeste brasileiro, região que concentra mais de 80% da produção nacional de melão. Epidemias de míldio são referidas, constantemente, causando severas perdas na produção de frutos. O efeito da doença no teor de sólidos solúveis totais (SST), importante fator de aferição qualitativa do fruto, foi determinado em meloeiro do tipo amarelo inodoro sob condições naturais. Intensidades variáveis da doença foram obtidas, variando-se o período de cultivo e a aplicação de fungicidas recomendados para o controle (tiofanato metílico + chlorotalonil, metalaxyl + mancozeb e oxicloreto de cobre). Os resultados revelaram que o aumento da severidade da doença resulta numa acentuada redução no teor de SST, proporcionando decréscimos de até 49%. Entre os parâmetros usados para se estimar os efeitos relacionados à doença, a área abaixo da curva de progresso da doença foi o mais eficiente, principalmente quando a pressão da doença aumentou.

Virulence Characterization of Single-Zoospore Isolates of Sclerospora graminicola from Pearl Millet.

Sclerospora graminicola, the causal agent of downy mildew in pearl millet, is well-known for variation in its virulence pattern. Nine single-zoospore isolates (Sg 026-Z-1 to Sg 026-Z-9) derived from an oosporic isolate Sg 026 from a pearl millet F1 hybrid cultivar Nath 4209 grown in a farmer's field in a village, Veelad, in Maharashtra state, India, and three controls (Sg 026, Field-1, and Field-2) were evaluated for their virulence in two experimental runs in a greenhouse. The isolates were maintained on pot-grown seedlings of a highly susceptible pearl millet line, 7042S, in a greenhouse through asexual (sporangial) generations. Pot-grown seedlings of six pearl millet potential differential lines/cultivars (7042S, NHB 3, MBH 110, ICMH 451, 843B, and 852B) were spray-inoculated with a sporangial suspension (5 × 105 sporangia ml-1) and maintained in a greenhouse at 25 ± 2°C. Data were recorded for latent period (days) and disease incidence (%), from which a virulence index (incidence × latent period-1) was calculated to quantify disease-causing potential of isolates. Results indicated significant variation in latent period, incidence, and virulence index among isolates. The isolates were classified into four distinct pathotype groups based on their virulence indices on six pearl millet lines. Because of the significant variation for virulence in the S. graminicola population infecting Nath 4209, it is recommended that the hybrid be regularly monitored for downy mildew infection in farmers' fields, and be replaced by a resistant cultivar that is genetically unrelated to the parental lines of Nath 4209. This will help delay or avoid development of downy mildew epidemics and the resulting heavy loss to pearl millet farmers in the region.

Weather and Downy Mildew Epidemics of Hop in Washington State

Relationships between weather and downy mildew epidemics of hop in the Yakima Valley of Washington State over a 38-yr period were examined with linear discriminant and logistic regression analyses. A linear discriminant function with minimum temperature in April X the total number of rainfall days in April and May, an indicator variable for the presence of an epidemic the preceding year, and the minimum temperature in May X the number of occurrences of three or more consecutive rainfall days in May correctly classified the epidemic status for 92% of the years. The percentage of years with mild epidemics correctly classified (specificity) and years with severe epidemics correctly classified (sensitivity) for a 10-yr test data set were 86 and 100%, respectively [...]

Spatial Pattern of Downy Mildew in Hop Yards During Severe and Mild Disease Epidemics

Hop plants in hills in three or four hop yards were assessed visually during severe and mild epidemics of downy mildew in the Yakima Valley of Washington. Several methods of analysis were used to investigate the spatial pattern of hop plants systemically infected with Pseudoperonospora humuli during the two types of epidemics. When disease incidences were low during the mild epidemics and early in the development of the severe epidemics, distributions associated with aggregation of disease provided a good representation of the frequency distribution of systemically infected shoots (...)

Effect of Downy Mildew Epidemics on the Seasonal Carryover of Initial Inoculum in Hop Yards

Effet de la lutte contre Pseudoperonospora humuli avec le metalaxyl sur l'incidence de la maladie chez le houblon l'annee suivante

Weather Factors Affecting Downy Mildew Epidemics of Hops in the Yakima Valley of Washington

Weather Factors Affecting Downy Mildew Epidemics of Hops in the Yakima Valley of Washington

Ecological factors limiting epidemics of hop downy mildew in arid climates

This study reports on factors affecting the development of hop downy mildew during the dry, hot season in the Sacramento Valley, as well as on factors contributing to the paucity of disease development in apparently favorable conditions. In flood-irrigated commercial hop yards, systemically infected shoots (which can be sources of inoculum) were present until late in the growing season. Dew and guttation fluid appeared on leaves and stems at the base of the plants after irrigation, but only a few lesions developed on the leaves. Data from Hirst spore traps, used to monitor the concentration of sporangia near systemically infected shoots, indicated a diurnal pattern of sporangial release. Sporangia were released with each sudden lowering of RH under laboratory conditions, and a more rapid lowering resulted in greater release. Sporangial release in the field coincided with RH drop. Most of the sporangia were released in the early morning during evaporation of dew and guttation fluid, and few sporangia were collected in the afternoon, evening, or night. Longevity of viable sporangia was dependent on RH. Relative humidity below 60 per cent killed sporangia in less than 3 hours. Even in well-irrigated yards the ambient RH remained below 60 per cent for about 10 hours each day; most sporangia do not survive throughout the day in such an environment. A few sporangia do survive on hop leaf surfaces through favorable RH conditions, and they initiate infection during the following dew period. Sporangia survived better on the lower surface than on the upper surface because of higher RH. Sporangia exposed to temperatures of 36, 39, and 42°C had delayed germination when subsequently placed in environments optimum for germination (temperatures of exposed leaves in the yards occasionally exceed 33°C during the day). The studies indicate that both inoculum and free moisture are present in the yards and can establish new infections, but that they do not occur concurrently long enough to start epidemics. In the event of summer rains, however, even these few infections can provide sufficient inoculum for an epidemic.