Roland Sigvald

Forecasting Sclerotinia stem rot in spring sown oilseed rape

Abstract A new forecasting model for predicting the need for fungicide applications for Sclerotinia stem rot on oilseed rape is presented. Logistic regression was used for calibration and receiver operating characteristic (ROC) curves were used to compare different models. Data from about 800 fields collected during a 10-year period were used in the analysis. The new forecasting model is a risk point table based on field specific data and precipitation. Six factors that affect Sclerotinia infection (number of years since last oilseed rape crop, disease incidence in last Sclerotinia host crop, crop density, rain in the last 2 weeks before flowering, weather forecast and regional risk for apothecium development) are given points with regard to the risk of heavy infestation. The new model was tested in 48 fields during 1996 and 30 fields during 1997 and predicted the need for fungicide applications with high accuracy. At a given risk point threshold, spraying recommendations were given in 75% of the fields that needed fungicide application and in 16% of the fields that did not need to be sprayed. If a lower threshold was used, spraying was recommended in more fields in need of spraying but also in more fields that did not need to be sprayed.

Calibration and verification of risk algorithms using logistic regression

The use of logistic regression is proposed as a method of verifying and calibrating disease risk algorithms. The logistic regression model calculates the log of the odds of a binary outcome as a function of a linear combination of predictors. The resulting model assumes a multiplicative (relative) relationship between the different risk factors. Computer programs for performing logistic regression produce both estimates and standard errors, thus permitting the evaluation of the importance of different predictive variables. The use of receiver operating characteristic (ROC) curves is also proposed as a means of comparing different algorithms. An example is presented using data on Sclerotinia stem rot in oil seed rape, caused bySclerotinia sclerotiorum.

Relationship between aphid occurrence and spread of potato virus Y° (PVY°) in field experiments in southern Sweden

Field experiments were carried out in potato fields in 1975–1979 to study the natural spread of potato virus Y° from diseased potato plants (virus sources) to progeny tubers of adjacent healthy potato plants. The occurrence of alate aphids was studied by using yellow water traps (YWT) and the aphid population on the potato foliage was estimated by counting aphids on 100 randomly chosen leaves. There was a good correlation between the number of alate aphids trapped in YWT and spread of potato virus Y°. Rhopalosiphum padi were probably the main vector, but many other aphid species could also have contributed, e.g. Aphis fabae, A. nasturtii, A. frangulae, Phorodon humili and Hyperomyzus lactucae. There were only weak association between numbers of apterae on potato leaves and the spread of PVY°.

Progress in aphid forecasting systems

Potato virus Y and many other viruses of potatoes cause major economic losses to seed potato production in many countries. Potato virus Y, which is transmitted in a non-persistent manner, is one of the most important virus diseases of potatoes in many countries in Europe and especially in the northern regions.During the last decade there has been an increasing interest in developing methods for potato virus forecasting. The abundance of virus vectors is often estimated by yellow water traps (YWT), suction traps or field surveys. In Sweden the relationship between occurrence of alate aphids and the proportion of PVY infected progeny tubers has been studied since 1975. A dynamic simulation model for PVY has been designed for predicting the incidence of PVY. The simulation model describes a system which includes e.g., healthy and PVY diseased potato plants, different aphid species as virus vectors an their efficiency as virus vectors, the susceptibility of the potato crop according to mature plant resistance and date of haulm destruction. There was a good correlation between model output and samples of progeny tubers tested for PVY.

The relationship between sowing date, infestation and damage by the maize stalk borer, Busseola fusca (Noctuidae), on maize in Awassa, Ethiopia

Abstract The relationship between sowing date and the maize stalk borer, Busseola fusca (Fuller) in terms of infestation rate, and crop damage was studied using ten successive sowing dates between April and July in 1985 and 1986. Different degrees of infestation by B. fusca were recorded from all sowing dates. Early sowing in April and the early part of May had significantly lower infestations of first generation larvae. Levels of infestation by second generation larvae were significantly higher on late sowing dates. High infestation by second generation larvae appeared to be connected with the high biotic potential of the moths of the first, non‐diapause generation. Second generation larvae caused crop loss ranging from 22–5 to 100% while it was only 0–22.6% due to the first generation. In conclusion, delay of sowing later than April may result in serious crop yield loss due to the second generation larvae of B. fusca in Awassa.

A degree-day model for regional prediction of first occurrence of frit flies in oats in Sweden

Abstract A degree-day model for regional prediction of first occurrence of frit flies, Oscinella frit (L.), in oat fields in Sweden was developed. The model parameters were calibrated using water trap and temperature data from one province. Another set of data from the same and other provinces was used to validate the predictions of the model. Different methods of degree-day calculation, based on either air or soil minimum temperatures, were compared and the least variation of temperature sums associated with date of first occurrence was found using actual degree-days calculated from air temperature data. In the validation of the mode, median date of first occurrence could be predicted with an error of less than five days in 12 of 15 province-year combinations. The mean predictive errors differed between years and provinces and also, in one case, due to choice of weather station within region. Based on these results we conclude that the degree-day model predicts regional occurrence of frit flies with reasonably small errors and that it will be an important part in the development of a decision support system for control of frit flies.

Aphids on Potato Foliage in Sweden and their Importance as Vectors of Potato Virus Y0

Abstract The aphid population on potato plants was estimated by counting aphids on 100 randomly chosen leaves from upper, middle, and lower parts of the plant. This was usually done during a period of 14 days in mid-July in 5–10 potato fields in each of 5 to 7 regions in southern and central Sweden from 1975 to 1980. The aphid population on potato plants varied considerably between years and regions from 0.1 to 20 aphids per leaf in most areas and years. Aphis nasturtii and Aphis franqulae were the dominating aphid species and 99 to 100% of the apterous population consisted of these two species, but Myzus persicae, Macrosiphum euphorbiae and Aulacorthum solani were also present in small numbers. The association between the occurrence of apterous aphids on potato foliage and spread of potato virus Y0 was very weak.

Genetic analysis for some plant and fruit traits, and its implication for a breeding program of hot pepper (Capsicum annuum var. annuum L.).

Two separate field experiments were conducted on hot pepper in which the genetic, heritability and environmental components of variation for seven plant and fruit traits in 78 genotypes and gene effects for four fruit traits in six generations of five crosses were estimated. The objectives were to determine the variation and effects of genes controlling inheritance of plant and fruit traits, and to propose efficient breeding procedures for improving the crop. Analysis of variance in half-diallel crosses revealed the presence of total genetic variability for seven traits among the 78 experimental entries with an excess of over-dominance genes. The presence of unequal distributions of dominant genes with positive and negative effects was observed among the parents and indicated the need to be cautious while selecting hot pepper parents for breeding purposes. Significant variability was also revealed in environmental sensitivity among the 78 experimental entries for some traits along with high heritability, which could be an advantage for a plant breeder but provides less clear opportunities for an agronomist to achieve better plant and fruit traits. Progeny generations (F(1), F(2), B(1) and B(2)) were found to perform better for fruit traits than their parents (P(1) and P(2)). The presence of significant gene interactions indicated a polygenic inheritance of the fruit traits studied and the possibility of pyramiding favorable alleles in the required directions at different levels of progeny generations. Heterosis, backcrossing, multiple crossing and pedigree breeding methods with recurrent selection may facilitate simultaneous exploitation of the genetic components and gene effects obtained. Nevertheless, it is doubtful whether selection efforts within the current set of hot pepper parents would be beneficial to achieve superior fruit traits for developing new varieties.

Occurrence of barley yellow dwarf virus and cereal yellow dwarf virus in pasture grasses and spring cereals in Latvia

Abstract The relative frequency of three viruses causing barley yellow dwarf disease (BYD) was assessed in spring cereals and pasture grasses at two regions in Latvia in 2000–2002. A total of 2589 leaf samples (367 from spring oats, 743 from spring barley, 1479 from predominant grass species) were collected from 44 fields of spring oats, 84 fields of spring barley, and 26 pastures. We found that isolates of barley yellow dwarf virus-PAV (BYDV-PAV), barley yellow dwarf virus-MAV (BYDV-MAV) and cereal yellow dwarf virus-RPV (CYDV-RPV) were present in these samples of spring cereals and pasture grasses. The most common isolates of barley yellow dwarf virus were BYDV-PAV and BYDV-MAV in both grasses and cereals, but there was a great difference between years and regions. The proportion of BYD symptomatic cereal samples that reacted positively in TAS-ELISA test was 9 to 15%. The overall BYDV/CYDV incidence in pasture grasses ranged from 2 to 19%. The incidence of BYDV/CYDV infection was higher in Festuca elatior than in other grass species. Isolates of CYDV-RPV were rather rare: only found in Lolium perenne and Dactylis glomerata among six grass species tested and more frequently in barley than oats. This paper reports the first quantitative survey of selected BYD-causing viruses in spring cereals and pasture grasses in Latvia and in the Baltic states. We conclude that three selected virus species are prevalent in spring cereals and pasture grasses in Latvia, although with great variation between years. Further studies are needed to obtain knowledge of the most critical factors that determine these fluctuations.

Forecasting the influence of temperature increase on the development of the Colorado potato beetle [Leptinotarsa decemlineata (Say)] in the Wielkopolska region of Poland

Abstract The Colorado potato beetle (Leptinotarsa decemlineata) is probably the most dangerous pest species of potatoes. This study assessed the influence of expected climate warming on the number of Colorado potato beetle generations in the Wielkopolska region of Poland using two simulation models, SimLep 1 and NumoGen 1. Two types of data sets were utilised in the simulations: real meteorological data recorded in Winna Góra (52.2N; 17.4E) in the period 2002–2011 and virtual data obtained after transformation of the real data to include an increase in the recorded temperature values between 1 and 6°C. The threat to potato crops from Colorado potato beetle was found to increase in response to expected climate change. A temperature increase of 2°C above that recorded during the last few years would allow the development of two full generations of the beetle, and further temperature increases may even permit the occurrence of three generations.