Controlling potato late blight with systemic-protectant fungicide combinations of metalaxyl and mancozeb

Disease development was monitored weekly in field plots inoculated withPhytophthora infestans and treated with metalaxyl, mancozeb or metalaxyl/ mancozeb combinations. Late blight pressure was moderate to severe in 1981 and 1983 and was slight to moderate in 1982 and 1984. In 1982–84 all treated plots had significantly less foliar damage and late blight tuber rot than untreated plots. Use of metalaxyl alone resulted in the greatest increase in disease (up to 80% defoliation) in the latter part of the growing season. Under slight to moderate disease pressure, metalaxyl/mancozeb and mancozeb treatments provided significantly better disease control than metalaxyl. Under moderate to severe disease pressure, high levels of disease (65% defoliation) occurred by the end of the season with mancozeb and metalzxyl/mancozeb on 7- and 14-day spray schedules, respectively. However, significantly better control (about 20% defoliation) was obtained during early season, rapidly developing epidemic conditions when only three applications of metalaxyl/mancozeb were included in a 7-day mancozeb spray program.     

Potato Late Blight and Tuber Yield: Results from 30 Years of Field Trials

During 1983–2012, three field trials per year were performed in each of the three southernmost counties in Sweden to test different fungicide programmes aiming to control late blight, primarily in the very susceptible potato cultivar Bintje. A dataset with results from these field trials was used (i) to examine possible changes in the appearance and behaviour of late blight attack over the years, (ii) to investigate the relationship between late blight in foliage and tuber blight, (iii) to investigate the relationship between late blight and tuber yield and (iv) to identify any correlations between different variables in the dataset. Late blight reached epidemic proportions, i.e. 75% disease severity in the untreated control, in the majority of the field trials. The estimated first attack of late blight was earlier in many field trials after 1998. Differences between years and regions were great in terms of date of the first attack and how the attack developed during the season. For example, in 2002–2005 and 2007, the first attack occurred 40–55 days after planting (DAP), compared with 95–108 DAP in 1994 and 1996. In 1994, 2006 and 2009–2011, the attacks increased from first symptoms to 65% disease severity in the untreated control within 16–21 days, compared with 35–40 days in 1995, 1999 and 2002. The relationship between foliage late blight and tuber blight was weak with the best match found at high disease severity late in the growing season (r?=?0.33; p?<?0.001). The relationship between blight-free tuber yield and start of the first attack indicated a yield increase of 287 kg/day (R ²?=?0.27) for every day’s delay in first attack. Using the DAP for 65% disease severity in the untreated control improved the correlation (R ²?=?0.64) and indicated a yield increase of 534 kg/day for every day’s delay in first attack. The later the onset of attack, the higher the blight-free tuber yield in treated plots. In general, significant correlations were found between blight-free tuber yield, size fractions of tuber yield, date of first late blight attack, date of different degrees of disease severity, disease severity, date of treatment, treatment measures and maintenance.     

Metalaxyl and other fungicides for control ofPhytophthora infestans on potato in northwest Washington State

In northwest Washington State, fungicidal control of late blight caused byPhytophthora infestons was evaluated for three years. During 1977, a high rainfall year with supplemental sprinkler irrigations, all metalaxyl and chlorothalonil treatments reduced tuber rot at harvest, but only metalaxyl extended control of tuber rot after 4 months’ storage. During 1978, a high rainfall year, one application of metalaxyl sprayed over or dusted on potato seed pieces at planting or applied five times to foliage at 2 or 4 wk intervals gave excellent blight control equal to that with chlorothalonil applied to foliage every 2 wk. In 1979, a low rainfall year,P. infestons did not kill plants in untreated plots until October. Five applications of metalaxyl sprayed every 2 wk on foliage did not control foliage blight. However, metalaxyl reduced foliage blight when dusted on cut potato seed pieces. Mancozeb sprayed every 2 wk also gave excellent control. The possibility that metalaxyl applied to foliage is effective in late blight control only when washed into the soil by rainfall or irrigation for uptake by roots is discussed. Yields and percent U.S. No. 1 grade tubers were not significantly affected by any treatment in these 3-year trials.     

Monitoring metalaxyl resistance in populations ofPhytophthora infestans

Summary Samples of potato foliage infected withPhytophthora infestans were collected from the main potato growing areas in Ireland over a three-year period starting in 1981 when metalaxyl-based fungicides were no longer commercially available. Using the floating disc method, 75% of the isolates tested were found to be resistant to metalaxyl in 1981, 21% in 1982 and only 6% in 1983. The reliability of the test method and the sampling procedure to detect low levels of resistance is discussed.     

Effects of Mulch and Potato Hilling on Development of Foliar Blight (<Emphasis Type="Italic">Phytophthora infestans</Emphasis>) and the Control of Tuber Blight Infection

Foliar and tuber blight caused by Phytophthora infestans accounts for significant losses in potatoes in field and storage. Nevertheless, limited research has been published on the effects of cultural practices on late blight control. Field experiments were conducted in two years on Howard gravely loam soil in New York State to evaluate the effectiveness of mulching using oat straw and hilling in preventing tuber blight infection for cvs Allegany and Katahdin. Potato hilling and mulching had little effect on foliar blight development. The cultivar affected the disease development in the foliage, with cv. Allegany showing lower foliar late blight than cv. Katahdin. Tuber blight incidence averaged 25% for cv. Allegany and 3% for cv. Katahdin in hilled plots, while in the mulched plots the incidence of tuber blight averaged 33% for cv. Allegany and 10% for cv. Katahdin. The straw hay mulch was ineffective in tuber blight control. Tubers set at a soil depth of more than 7 cm had lower tuber blight incidence than shallow tubers set at a depth of less than 7 cm. In both years, hilling provided partial protection of tubers but its effectiveness was limited in the presence of favourable conditions for late blight development. Even though large hills had proportionally a lower tuber blight incidence than medium-sized hills, the difference between the different hill sizes was not significant. These studies suggest that the use of cultivars with foliage resistance to late blight in combination with cultural practices may partially reduce the incidence of tuber blight. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation for endorsement by Cornell University or the US Department of Agriculture.     

Potato late blight control and top desiccation when pesticides are applied with a controlled droplet and a conventional sprayer

Disease development was observed in field plots to which two different fungicides, each at two rates, were applied regularly with either a conventional or a controlled droplet sprayer. The type of fungicide did not affect disease severity, but more disease damage occurred in the controlled droplet sprayer plots than in conventional sprayer plots. Reduced rates of fungicide resulted in higher disease incidence ratings than recommended rates of fungicides by end of the spraying season. Lower marketable tuber yields and greater incidences of late blight tuber rot were related to foliar disease levels, the different sprayer types, and rates of applied fungicide. No significant differences in terms of potato top desiccation efficacy between the two sprayer types were obtained. In addition, total tuber yield and level of vascular discoloration were similar for both the controlled droplet and conventional sprayer.     

Potato yield losses due to early blight in Minnesota fields, 1981 and 1982

In both 1981 and 1982, 51 potato fields were surveyed every 7–10 days for early blight severity and fungicide use. Early blight severity data from each field were used as input to a regression model to estimate yield loss in that field. The model for early maturing potato cultivars was Percent Yield Loss=0.8183+0.6441* (% blight increment between days 56 and 66)+0.6102* (% blight increment between days 66 and 76)+1.3480* (% blight increment between days 76 and 86). The model for late maturing cultivars was Percent Yield Loss=2.1846-4.7734* (% blight on day 56)+0.7440* (% blight on day 76)+ 0.5676* (% blight on day 96). In both 1981 and 1982, early blight was found to be present in all fields of the survey. In 1981, early blight was estimated to have caused mean losses of 6.5% (Red River Valley), 3.7% (Central sands) and 2.3% (Southern muck). In 1982, losses from the same areas were 5.4%, 3.7% and 2.6% respectively. In 1981, individual county losses ranged from 2.3% for Freeborn to 12.9% for Marshall while in 1982 losses were from 2.0% for Freeborn to 6.7% for Clay. Although fungicide use data were incomplete, the county with lowest yield loss had the most fungicide applications.     

Potato cultivar response to late blight as affected by clonal selection andin vitro culture

Response to foliar late blight for Superior and Sebago was not affected by the two potato seed selection and multiplication methods. Kennebec and Russet Burbank potato plants derived throughin vitro tissue culture techniques had significantly more late blight damage on only 4 of 7 and 2 of 7 observation dates, respectively, than plants derived through a clonal selection system. In practical terms, the disease response differences between the two seed propagation methods were minimal. Similarly, disease response differences among the 2 and 3 years of field multiplication for Russet Burbank and Kennebec, respectively, did not demonstrate any significant disease response trends. In general,in vitro culture plots had slightly higher yields than clonal plots but only Kennebec had a significant yield response. The incidence of late blight storage rot was generally not significantly different but clonally selected Kennebec potatoes had significantly more disease than those produced throughin vitro culture.     

Relationship between resistance to late blight in potato foliage and tubers of cultivars and breeding selections with different resistance levels

Due to changes in the pathogen population, late blight of potatoes caused byPhytophthora infestans has become a more difficult disease to manage and there is a need for new potato cultivars with greater and more stable disease resistance. Field studies are required to identify and characterize host resistance to late blight in both foliage and tubers but, as epidemics vary from year to year, combining multi-year results can be difficult. Residual maximum likelihood (REML) was successfully used to analyze data which demonstrated good correlations between foliar and tuber responses to late blight (US-1 genotype). In addition, studies on post-harvest inoculated tubers provided more reliable disease responses than field studies, and the disease resistance with this method correlated well with foliar disease responses. This tuber test would be a useful method of pre-screening selections for disease response prior to testing in the field.     

Susceptibility of potato (Solanum tuberosum L.) foliage and tubers to the US8 genotype ofPhytophthora infestans

Late blight is an important disease of potato(Solanum tuberosum L.) worldwide, and therefore, many potato-breeding programs have prioritized the development of late blight-resistant potato cultivars. Although the emphasis has been to enhance foliar resistance, it is also necessary to evaluate tuber late blight resistance in new breeding lines and new sources of late blight resistance. We report here on the assessment of foliar and tuber resistance and the correlation between these aspects of resistance in a sample of Michigan State University potato breeding lines. Two MSU breeding lines had significantly less infected foliage than the susceptible check cultivars. Tuber susceptibility was significantly different (P < 0.05) only between the most susceptible and the least susceptible breeding lines/cultivars. Foliar and tuber susceptibility to potato late blight were not correlated as low tuber susceptibility was associated both with extremely low (e.g., MSG274-3) and high (e.g., MSE202-3Rus) foliar susceptibility.     

Interactive effects of fungicide programs and nitrogen management on potato yield and quality

The use of azoxystrobin (Quadris) for early blight control often results in potato vines remaining greener later in the growing season. This observation has lead to the suggestion that nitrogen (N) fertilizer could be applied at lower rates when azoxystrobin is used in a fungicide program since high N rates are sometimes used to help manage early blight. The objectives of this study were to evaluate the effectiveness of azoxystrobin for control of early blight under various N fertility management regimes and to determine if azoxystrobin affected plant N fertilizer requirements. Plots were established in 1999 and 2000 at Becker, MN, on a Hubbard loamy sand as a complete factorial, split-plot arrangement using a randomized complete block design with four replications. Fungicide treatments were used as the whole plot factor and included an untreated control, chlorothalonil, and azoxystrobin rotated with chlorothalonil. Fertility treatments were used as the sub-plot factor and included N applied at three levels (170, 250, and 340 kg ha^?1) and two timings (all prehilling or pre- and post-hilling). Early blight was problematic in 1999 and both early and late blight were severe in 2000. Significant interactions were observed between fungicide and fertility treatments for disease control. Control of diseases with fungicides was generally more effective at higher N rates regardless of N application timing. When azoxystrobin was used in the fungicide program, N rate was not as critical in managing foliar disease. In 1999, an interaction was not observed between N rate and fungicide treatment for yield. Yields increased with increasing N rate regardless of fungicide program. However, a significant interaction did occur between N rate and fungicide treatment in 2000 where yields decreased linearly in the control with increasing N rate, increased linearly with N rate using chlorothalonil, and did not respond significantly to N rate using azoxystrobin/chlorothalonil. Results of this study indicate that interactions between the incidence of early/late blight disease and N requirement for potato may depend on the rate of vine death and conditions affecting tuber maturity. At equivalent N rates, post-hilling N tended to depress yield regardless of fungicide program, suggesting that under the conditions of this study late season N is not an effective practice for managing foliar diseases such as early or late blight.     

Effects of pest and soil management systems on potato diseases

Long-term cropping systems research is important in order to reduce production costs, to control crop pests, and to optimize the sustainability of agro-ecosystems. Soil amendment use, improved disease management practices, and careful cultivar choice are some of the potential components for improving potato production systems. This research was conducted in long-term cropping systems plots in order to evaluate the impact of soil amendments, pest management practices, and cultivar on foliar and soil-borne potato diseases and to assess the relationships of soil and pest management practices to disease levels and soil microbial activity. Fungicide applications for management of foliar diseases varied between the pest management systems (e.g., biological, reduced input, and conventional). Incidence of potato foliar diseases was quantified five times during the cropping season. The impact of soil amendment and pest management practices on soil microbial activity and tuber-borne diseases was also investigated. Low incidences of foliar and selected soil-borne diseases were recorded. Disease levels varied between years, cultivars, pest management, and soil amendments. Significant differences between cultivars were detected for early blight, white mold, and black dot. The cultivar Superior had higher incidence of white mold and black dot, while cv Atlantic had higher early blight incidence. Pest management system significantly affected foliar early blight incidence in 1998, but not in 1997. Pest management system did not affect late blight, white mold, or black dot incidence, or tuber disease incidence in either year. The addition of soil amendments significantly impacted tuber black dot incidences. Microbial activity responded to increasing temperature as the season progressed and was significantly enhanced by the addition of manure and compost soil amendments; however, lower disease incidence was not associated with increased microbial activity. While pest management practices were not major determinants of disease levels in these experiments, the results show that soil amendments can increase incidence of selected tuber diseases and microbial activity in soils.     

Two Cycles of Recurrent Maternal Half-Sib Selection Reduce Foliar Late Blight in a Diploid Hybrid Solanum phureja-S. stenotomum Population by Two-thirds

Foliar late blight, caused by Phytophthora infestans, is an important disease problem worldwide. Foliar resistance to late blight was found in a hybrid population of the cultivated diploid species Solanum phureja-S. stenotomum (phu-stn). The objective of this study was to determine if resistance to late blight could be improved by recurrent maternal half-sib selection in the phu-stn population. Four clones from each of 72 maternal half-sib families plus the check cultivar ‘Atlantic’ were evaluated for late blight resistance in replicated field trials in Pennsylvania. The most late blight resistant clone from each of the 72 families was randomly intermated to advance the population. The cycle 1, 2, and 3 populations were evaluated in 1996–1997, 2003–2004, and 2009–2010, respectively. Percent infected foliage was recorded several times towards the end of the growing season and used to compute area under the disease progress curve (AUDPC). Mean AUDPC was 652 (cycle 1), 556 (cycle 2), and 276 (cycle 3), whereas the mean AUDPC of ‘Atlantic’ ranged from 775 to 863 each cycle. Narrow-sense heritability for resistance was estimated as 0.78, 0.77, and 0.80 in cycles 1, 2, and 3, respectively. Recurrent maternal half-sib selection continues to improve the levels of resistance to late blight in this diploid population with no adverse effect on the amount of additive genetic variance. Additional improvements for late blight resistance are likely to occur if this approach is continued.     

Two decades of monitoring irish populations of<Emphasis Type="Italic">Phytophthora infestans</Emphasis> for phenylamide resistance

Summary Between 1981 and 1998,Phytophthora infestans populations in 878 potato crops in Ireland were monitored for phenylamide resistance. The incidence of resistance was very high in 1981 following intensive use of metalaxyl for blight control from 1977 onwards. It decreased rapidly in the absence of phenylamide use between 1981 and 1984. Following the reintroduction of phenylamide based fungicides in 1985, there was a rapid increase in phenylamide resistance following the widespread curative use of these fungicides. From 1990 onwards there was greater adherence among growers to the use of phenylamides in accordance with an antiresistance strategy and resistance distribution again decreased. During the 18-year period of the study, resistance was always lower at the beginning of the season and increased as the season progressed. Following the reintroduction of the phenylamide based fungicides in 1985 there was a steady increase in phenylamide use up to 1986 followed by a continuous decrease in use up to 1990. Since then there has been a steady increase in phenylamide use. They are currently used on some 60% of crops in Ireland and are still providing acceptable levels of disease control.     

Control of early blight in eastern and southcentral Idaho

Early blight foliage infection can be adequately controlled in eastern and southcentral Idaho with relatively few applications of fungicide if the initial application is properly timed. The initial application should occur near the time when secondary infection is beginning in the field. Spray schedules which began when the plants were small and continued every 12–14 days throughout the season were no more effective in controlling the disease than were three applications properly timed. Early season applications were of no apparent economic value in the overall blight control program.     

The effect of autumn fungicide application on spring dry matter accumulation in perennial ryegrass cv. S24

Fungicide applied to plots of perennial ryegrass cv. S24 in the autumn of 1981 significantly increased tiller number and total dry matter accumulation in February and March of 1982. However, fungicide application in the autumn of 1982 failed to increase tiller number or dry matter accumulation in the spring of 1983. It is suggested that fungicide application in autumn 1981 prevented the death of tillers caused by fungal attack after snow covered the plots in December 1981.     

Area-wide suppression of boll weevil (Coleoptera: Curculionidae) populations in Nicaragua

Between 1982 and 1984, the Nicaraguan Ministry of Agricultural Development and Agrarian Reform (MIDINRA) used an area-wide network of trap crops of cotton from February to August to suppress boll weevils (Anthonomus grandis Boheman) on 17450 ha (1982–1983) and 35 200 ha (1983–1984) of cotton land in the Leon region of Nicaragua. In the 1982–1983 season the programme reduced boll weevil infestations (expressed as weevil-days) 89% in Zone II (normal rainfall area) when compared with untreated controls. This suppression of adult weevils resulted in a reduction from 18 to 10 seasonal applications of methyl parathion for weevil control. The programme reduced weevil insecticide costs per kg of raw cotton produced by an area-wide average of 43%. In the 1983–1984 season the programme reduced weevil-days by an area-wide 36% in commercial fields. In Zone II, trap cropping reduced by 40% the quantity of insecticides applied for weevil control. Weevil control costs per kg of raw cotton produced were reduced by 44%. Non-significant cost reductions for weevil control in Zone I may have been due to early drought, cattle grazing on trap crop areas, or inadequate management of trap crops. The 2-year programme was the largest mobilization of public and private resources for pest management ever undertaken in Nicaragua, and gives practical evidence that suppression of between-season boll weevil populations can form the basis for more profitable control of this key pest in the commercial cotton season.     

Estimating potato yield responses from chemical control of early blight in Minnesota

The effect of early blight on tuber yield of two potato (Solanum tuberosum L.) cultivars (Russet Burbank and Norland) was evaluated using number and timing of applications of three chemicals to generate different disease epidemics in 1981 and 1982. Under the non-irrigated farming conditions in Minnesota, there were no significant differences among captafol, triphenyltin hydroxide and maneb-Zn in blight control to increase yield. Of the five spray schedules tested, a 7–10 day spray schedule initiated when blight severity exceeded 0.01%, and a 7–10 day schedule initiated when blight was found above the lower third of the plant, generally gave significant, positive yield responses with all three chemicals. At Rosemount in 1982, the greatest yield increases (relative to unsprayed) obtained were 34.9% and 48.6% in total tuber yields for Norland and Russet Burbank, respectively. At Rosemount in 1982, the highest yield increases (relative to unsprayed) obtained were 92.2% and 56.6% in total tuber yields for Norland and Russet Burbank respectively. At Grand Forks in 1981, the highest yield increases were 126.8% and 34.6% in total tuber yields for Norland and Russet Burbank respectively. Across all three experiments, maximum yield losses (percentage of [[yield of ‘healthy’—yield of unsprayed check]/yield of ‘healthy’’]) were 58.4% (US #1 tubers) and 55.9% (total tubers) for cv. Norland and 34.4% (US #1 tubers) and 36.2% (total tubers) for cv. Russet Burbank.     

Assessment of mating types and resistance to metalaxyl of Canadian populations ofPhytophthora infestans in 1997

The distribution of A1 and A2 mating types, and of metalaxyl-sensitive (MS), metalaxyl-intermediate (MI), and metalaxyl-resistant (MR) strains ofPhytophthora infestans, the causal pathogen of late blight, were determined in Canada in 1997. Potato plants or tubers and tomato plants or fruits, suspected to have late blight infection, were received from seven Canadian provinces in 1997, and were assessed for the presence ofP. infestans. Among 220 samples suspected to have late blight,P. infestans was recovered from 132 samples (60%) received from British Columbia (BC), Manitoba, New Brunswick, Prince Edward Island, and Quebec. From the 132 samples, 364 isolates were obtained and tested for their mating type and response to metalaxyl. Populations ofP. infestans within Canadian provinces were homogeneous for mating types, except within BC, where populations comprised both A1 and A2 mating types both within fields and sometimes within samples. The A2 mating type was found in all provinces where late blight was present. The A1 mating type was found only in Manitoba (1 isolate) and in BC (38 isolates). Unlike the old A1 isolates, those found in BC in 1997 were highly resistant to metalaxyl. These A1 isolates are even more resistant than A2 isolates. Based on assays with 10 ug.ml^?1 of metalaxyl, 14% of all tested isolates were MS, 69.8% were MI and 16.2% were MR. In addition, all provinces contained MS, MI, and MR isolates. No specific mating type or metalaxyl sensitivity level were specific to tomato or potato.     

Identification of mating types and metalaxyl resistance in North American populations ofPhytophthora infestans

The A² mating type ofPhytophthora infestans was first reported in the United States in 1990. Concurrently,P. infestans strains resistant to metalaxyl ere found in the Pacific Northwest. Collaborative surveys were undertaken during 1991–1993 to investigate the frequency of occurrence of A² mating types and metalaxyl resistant strains in populations ofP. infestans isolated from outbreaks of late blight in potato and tomato crops in North America.In vitro testing indicated that isolates from the northeastern U.S. and Atlantic Canada were primarily (52/55) metalaxyl sensitive and all were A¹ mating types. Among 85 isolates from late blight epidemics in Florida and Texas, greater than 61% were both metalaxyl resistant and A² mating type. Metalaxyl resistance and A² mating types were identified also in a few tomato isolates from North Carolina. Although the majority of 134 isolates from the Pacific Northwest (British Columbia and Washington) were metalaxyl resistant, only 2 isolates from Washington were A² mating types. Among 111 isolates from 2 sites in central Mexico, 63% and 77% were both metalaxyl resistant and A² mating types. The data indicate also a higher frequency of metalaxyl resistance in A² isolates, than in A¹ isolates, among isolates from Florida and Texas. Highest metalaxyl resistance levels were found, however, in A¹ isolates from California, where no A² isolates were recovered.