Sunflower disease models
The fungus causing Downy mildew in sunflowers is known under the names Plasmopara halstedii or Plasmopara helianthi. The fungal pathogen complex infects a wide range of genera in the family Asteraceae, including wild and cultivated species of Helianthus. The disease is present wherever sunflowers are grown.
Starting from a single oospore that germinates and gives rise to a zoosporangium, zoospore differentiation and release are the subsequent steps of development. In the presence of free water, the zoospores move to infection sites (root, hypocotyl) soon if available. Following encystment and germ tube elongation (the latter usually terminates in an appressorium against a host cell) the fungus develops an infection structure (infection peg) for direct penetration. Under experimental conditions, it was demonstrated that germ tubes do not usually form appressoria in water but they do so in the presence of host cells (Gray et al., 1985). After penetration, the fungus grows intracellularly and then intercellularly, and once being established in a susceptible host (compatible) it starts to colonize the entire plant systemically by growing preferably toward the shoot apex and, to a lesser extent, in the direction of the root. When conditions are favourable, asexual sporulation takes place on affected leaves and occasionally on below-ground tissues. Fully developed sporangia disseminate by wind and, since they are short-lived and sensitive to drought and direct sunshine, their survival depends on the current weather situation. Oospores are also produced in infected plant parts, primarily in root and lower stem tissues, whereas leaves and upper plant parts, except seeds, are free from these resting spores (Sackston, 1981; Virányi, 1988; Onan and Onogur, 1991). The most susceptible stage of host development is between germination and emergence (Meliala et al. 2000).
Survival and Source of Inoculum
With respect to the primary infection, P. halstedii is a soilborne pathogen. Its oospores serve as primary inoculum to underground tissues of young sunflower seedlings. It may also be windborne, causing secondary infection of leaves and/or inflorescence. If the latter is the case, the fungus might also be seedborne: the affected seeds carrying mycelium and/or oospores internally. Oospores develop mainly in root and lower stem tissues of mildewed plants, with or without visible symptoms and, with plant residues of the preceding sunflower crop, they come into the soil. Oospores are long-lived and are able to survive for at least 6-8 years (Sackston, 1981; Virányi, 1988). It is generally thought that oospores mainly germinate under wet conditions. However, only a few results on the germination dynamics have been available so far. A low-temperature shock prior to wetness and the presence of host exudates released by roots were shown to enhance the germination process (Delanoe, 1972). In another report (Spring & Zipper 2000), no such temperature effects could be observed and freshly developed oospores were reported to germinate spontaneously in water within a period of 10-30 days, but at a highly variable rate (1-17%).
However, secondary infection is considered as an important factor in the spreading of the disease in certain regions under favourable environmental conditions. Apart from the fact that secondary infection of inflorescence may give rise to latent infection of seeds by P. halstedii (Sackston, 1981), from local leaf lesions the fungus is able to proceed and grow into the stem causing systemic infection (Spring 2001).
The nature of the inoculum (oospore or zoospore), weather variables (relative humidity, temperature), infection site (age of tissue), as well as cultivar reaction are factors that influence or determine the infection process, disease incidence, and severity. Zoospores, originating from either sexual or asexual sporulation, require free water for retaining viability and capability of moving toward infection sites. Consequently, rainfall or intensive irrigation will be a prerequisite for the initiation of infection. It was shown by several studies that if there was enough rain or corresponding water supply during the first two weeks after sowing, the incidence of primary infection from the soil increased. However, the duration of time that favours infection is relatively short and even susceptible sunflowers become resistant with age (Sackston, 1981). Tourvieille et al. (2008a) found that the risk of downy mildew attack appeared greatest if there was heavy rainfall when sunflower seedlings were at their most susceptible stage, whereas heavy rainfall before sowing or after emergence had no effect on the percentage of diseased plants. Göre (2009) that low temperature and extensive spring rains in approximately 85% yield loss and lower quality of sunflower production in the Marmara region of Trace. Besides environmental conditions, disease intensity may also be influenced by the aggressiveness of the pathogen population. Sakr et al. (2009) were able to differentiate the two pathogen strains in terms of their aggressiveness based on the population’s latent period and sporulation density.
P. halstedii has been found to occur in sunflower seeds from naturally infected plants, either as mycelium or oospores (Novotel’nova, 1966). Doken (1989) reported that the mycelium was only found in the testa and in the inner layer of the pericarp; it was absent from the embryo. Following artificial inoculation, Cohen and Sackston (1973) confirmed that sunflower buds inoculated with P. halstedii became systemically infected and produced infected seeds. Oospores were observed in seeds of inoculated and naturally infected plants in the field. Other records of seed infection are known from Iran (Zad, 1978), Turkey (Döken, 1989) and Germany (Spring, 2001). The fungus usually invades the ovary and the pericarp but fails to grow into the embryo (Novotel’nova, 1966; Döken, 1989). Seed infection regularly occurs in systemically infected plants if they survive up to the flowering stage. In such cases, the development of the embryo is often retarded or inhibited. Moreover, such plants are dwarf and will seldom be harvested. They may increase the local stock of oospores in a field, but for the seed-derived long-distance dispersal of the pathogen they appear to be less important than seeds from late infected symptomless plants (Spring, 2001). The latter type of infection is very dependent on the weather conditions during the flowering process. Thus in dry years the number of pathogen-contaminated seeds is very low and may not exceed several in one thousand, but may be much higher after a cool and humid period in June/July. For example, Spring (2001) found that close to 10% of seeds from a field in Germany were contaminated and Döken (1989), under favourable experimental conditions, observed fungal structures in 28% of the seeds examined.
Effect on Seed Quality
Sunflower seeds produced in downy mildewed plants are either under-developed, colourless or, rarely, they look healthy. Even in the latter case, such infected seeds are of poor quality; they produce abnormal seedlings and the germination rate is low (Döken, 1989).
For further information have a look at the homepage of CABI.
Model in FieldClimate
Used sensors: soil temperature, precipitation, leaf wetness, air temperature and relative humidity.
We start to calculate the infection progress, when temperature is between 6 and 32°C with optimum from 18 to 24°C and soil temperature are above 10°C. Further on wet conditions are favourable for the disease (rain event, relative humidity above 70%).
Makrosporangia are formed at soil temperatures above 10°C and precipitation (rel. humidity more than 70%). Reset is if relative humidity falls below 50%.
If Makrosporangia are fully developed – the calculations for a soil infection or an air infection (in the graph called primary infection) start (under leaf wetness conditions).
Sporangia are formed under humid conditions (more than 95% r.h.), in darkness and temperatures above 12°C. Reset is during the day and when sporangia are not fully developed.
If Sporangia are fully developed the calculation of the secondary infection starts in dependance of the air temperature.
In the Graph you see at the end of April a long-lasting humid period, which lead to the formation of macrospores and a soil infection (primary root infection). An air infection (called primary infection here) was not determined on the first of May, but conditions have been favourable so it was determined on the 2nd of May. If sunflower seedlings are in a sensitive stage at that time (just sown) control measurements have to be into account ( prophylactic systemic fungicides, mostly phosphoric acids).
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Carpogenic germination of sclerotia is stimulated by periods of continuous soil moisture. Apothecia are formed on the soil surface from which ascospores are released into the air. Infection of most crop species is principally associated with ascospores but direct infection of healthy, intact plant tissue from germinating ascospores usually does not occur. Instead, infection of leaf and stem tissue of healthy plants results only when germinating ascospores colonize dead or senescing tissues, usually flower parts such as abscised petals, prior to the formation of infection structures and penetration. Myceliogenic germination of sclerotia at the soil surface can also result in colonization of dead organic matter with subsequent infection of adjacent living plants. However, in some crops, for example sunflower myceliogenic germination of sclerotia can directly initiate the infection process of the roots and basal stem resulting in wilt. The stimulus for myceliogenic germination and infection in sunflower is not known but likely depends on nutritional signals in the rhizosphere derived from host plants.
The infection process
The infection of healthy tissue depends on the formation of an appressorium, which may be simple or complex in structure depending on the host surface. In most cases, penetration is directly through the cuticle and not through stomata. Appressoria develop from terminal dichotomous branching of hyphae growing on the host surface and consists of a pad of broad, multi-septate, short hyphae that are orientated perpendicular to the host surface to which they are attached by mucilage. Complex appressoria are often referred to as infection cushions. Although earlier workers considered the penetration of the cuticle to be a purely mechanical process there is strong evidence from ultrastructural studies that enzymatic digestion of the cuticle also plays a role in the penetration process. Little is known about S. sclerotiorum cutinases, however, the genome encodes at least four cutinase-like enzymes (Hegedus unpublished). A large vesicle, formed at the appressorium tip prior to penetration, appears to be released into the host cuticle during penetration. After penetration of the cuticle, a subcuticular vesicle forms from which large hyphae fan outgrowing over and dissolving the subcuticular wall of the epidermis.
Infection by enzymatic degradation of the epidemic cells: Oxalic acid works in concern with cell wall degrading enzymes, such as polygalacturonase (PG), to bring about the destruction of host tissue by creating an environment conducive for PG attack on pectin in the middle lamella. This, in turn, releases low molecular weight derivatives that induce the expression of additional PG genes. Indeed, overall PG activity is induced by pectin or pectin-derived monosaccharides, such as galacturonic acid, and is repressed by the presence of glucose. Examination of the expression patterns of individual Sspg genes has revealed that the interplay among PGs and with the host during the various stages of infection is finely co-ordinated. (Dwayne D. Hegedus *, S. Roger Rimmer: Sclerotinia sclerotiorum: When ‘‘to be or not to be’’ a pathogen? FEMS Microbiology Letters 251 (2005) 177–184)
Looking for Climate Conditions for Infection of S. sclerotiorum has to take consideration of the apothecia formation, the sporulation, the direct infection by apothecia (even if it does not take place very frequent) and the infection from established mycelia by encymatic degradation of the epidemic cells. Apothecia formation and sporulation takes place if a rain of more than 8 mm is followed by a period of high relative humidity lasting longer than 20 hours at optimum temperature of 21°C to 26°C.
Direct Infection by Apothecia can be expected after a leaf wetness period followed by 16 hours of relative humidity higher than 90% under optimum 21°C to 26°C (“appressoria infection”). Wheras saprophytic growth followed by encymatic degratation of the epidermic cells (“hydrolytic infection”) can be expected under a slightly lower relative humidtiy of 80% lasting for a period of 24 hours under optimum conditions of 21°C to 26°C.
- Lumsden, R.D. (1976) Pectolytic enzymes of Sclerotinia sclerotiorum and their localization on infected bean. Can. J. Bot. 54,2630–2641.
- Tariq, V.N. and Jeffries, P. (1984) Appressorium formation by Sclerotinia sclerotiorum: scanning electron microscopy. Trans. Brit. Mycol. Soc. 82, 645–651.
- Boyle, C. (1921) Studies in the physiology of parasitism. VI. Infection by Sclerotinia libertiana. Ann. Bot. 35, 337–347.
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Practical Use of the Sclerotinia Model
The White Leg Infection Model shows the periods when the formation of apothecia are expected. If these periods are coinsitent with the flowering period of rape seed or canola we have to expect S. sclerotiorum infections during a moist period. The spores formed in the apothecia might be available for one to several days. The opportunity of infections is indicated by the calculation of the infection progress for direct or indirect infections by appressoria or enzymatic cell wall degradation. If the infection progress line reaches 100% an infection has to be assumed. These infections should be covered preventative or a fungicid with a curative action against S. sclerotiorum has to be used.
Grey Mould (Botrytis cinerea) is a devastating disease with a high economic impact in production. B. cinerea infects the flowers and the fruits close to maturity.
The fungal pathogen has a very broad host range, infecting more than 200 different hosts. Fungal growth exists saprophytically and parasitic.
On Sunflowers the pathogen causes a grey mold on the head and stem. There while the leaves start to dry out. These symptoms occur during the maturation of kernels on the head. Brown spots on the backside are seen. These spots are covered by the fungal mycelium and spores, giving the appearance of a powdery. Spores are able to be spread during wet weather conditions.
Black sclerotia deprived of medulla appear on the crop debris after harvesting or directly on the plants if they are harvested too late.
The fungus overwinters during winter on the soil surface or in the soil as mycelium or sclerotia. In springtime, the overwintering form starts to germinate and produce conidia. These conidia are spread by wind and rain and infect new plant tissue.
Germination is possible at relative humidity over 85%. The optimal germinating temperature is 18°C. The fungal pathogen can reproduce multiple times.
Control options: Seed control can protect plants of damping- off. Chemical control is difficult due to the resistance of the pathogen. Therefore attempts are made for natural control strategies with Trichoderma harzianum.
Conditions for modeling infection
B. cinerea infections are related to free moisture. Therefore in open field production leaf wetness, which is a good indicator, is determined.
Bulger et al. (1987) studied the correlation of leaf wetness periods during flowering and the occurrence of grey mold on the fruits. They found that for a higher risks of infection at 20°C a time period of longer than 32 hours of leaf wetness is needed. At lower temperatures, the leaf wetness periods have to be longer for infection of the disease.
FieldClimate is indicating risk of Botrytis cinerea on the base of leaf wetness periods and the temperature during these periods.
The graph below shows the duration of wet leaves in dependence of the actual temperature needed for a Botrytis infection. If the risk is higher than 0 every leaf wetness period longer than 4 hours will increase the risk by the same relation.
A day with a leaf wetness period shorter than 4 hours is assumed to be a dry day and will reduce the risk by 20% of the actual value.
Practical use of the Grey Mould Model: The model indicates periods with a risk of a Botrytis infection. This risk period during the bloom of strawberry will lead to infected fruits. As longer the risk period lasts and as higher the risk is as higher is the probability and the number of infected fruit. The risk, which can be taken into consideration, depends on the market. Growers, which are selling their fruits to the supermarket will not take any risk, knowing that they are not able to sell infected fruits. While growers, who sell their fruits directly to the people are able to take a higher risk.
- Bulger M.A., Ellis M. A., Madden L. V. (1987): Influence of temperature and wetness druation on infection of strawberry flowers by Botrytis cinerea and disease incidence of fruit originating from infected flowers. Ecology and Epidemiology; Vol 77 (8): 1225-1230.
- Sosa-Alvarez M., Madden L.V., Ellis M.A. (1995): Effects of temperature and wetness duration on sporulation of Botrytis cinerea on strawberry leaf residues. Plant disease 79, 609-615.
TOMCAST (TOMato disease foreCASTing) is a computer model based on field data that attempts to predict fungal disease development, namely Early Blight, Septoria Leaf Spot and Anthracnose on tomatoes. Field placed data loggers are recording hourly leaf wetness and temperature data. This data where analysed over a 24 hour period and may result in the formation of a Disease Severity Value (DSV); essentially an increment of disease development. As DSV accumulate, disease pressure continues to build on the crop. When the number of accumulated DSV exceed the spray interval, a fungicide application is recommended to relieve the disease pressure.
TOMCAST is derived from the original F.A.S.T. (Forecasting Alternaria solani on Tomatoes) model developed by Drs. Madden, Pennypacker, and MacNab? at Pennsylvania State University (PSU). The PSU F.A.S.T. model was further modified by Dr. Pitblado at the Ridgetown College in Ontario into what we now recognize as the TOMCAST model used by Ohio State University Extension. DSV are: A Disease Severity Value (DSV) is the unit of measure given to a specific increment of disease (early blight) development. In other words, a DSV is a numerical representation of how fast or slow disease (early blight) is accumulating in a tomato field. The DSV is determined by two factors; leaf wetness and temperature during the “leaf wet” hours. As the number of leaf wet hours and temperature increases, DSV accumulate at a faster rate. See the Disease Severity Value Chart below.
Conversely, when there are fewer leaf wet hours and the temperature is lower, DSV accumulate slowly if at all. When the total number of accumulated DSV exceeds a preset limit, called the spray interval or threshold, a fungicide spray is recommended to protect the foliage and fruit from disease development.
The spray interval (which determines when you should spray) can range between 15-20 DSV. The exact DSV a grower should use is usually supplied by the processor and depends on the fruit quality. Following a 15 DSV spray interval is a conservative use of the TOMCAST system, meaning you will spray more often than a grower who uses a 19 DSV spray interval with the TOMCAST system. The trade off is in the number of sprays applied during the season and the potential for difference in fruit quality.
Studies have been initiated at Michigan Staate University to test the disease forecasting system, TomCast, for use in managing foliar blights on carrot. TomCast has been used commercially in tomato production, and has recently been adapted for use in disease management of asparagus. Processing carrots ‘Early Gold’ were planted with a precision vacuum seeder at the MSU Muck Soils Research Farm in three rows 18 inches apart on a raised bed that was 50 feet long. Carrot beds were spaced on 64 inch centers and inrow seed spacing was 1 inch. Each of the four replications of the experiment were located in separate blocks of carrots that consisted of 36 beds. Seventeen treatment beds 20 feet long were randomly placed in a checker board pattern in each replication. Treatments were applied with a CO2 backpack sprayer that was calibrated to deliver 50 gallons per acre of spray solution using 8002 flat fan nozzles. Treatments consisted of an untreated and different schedule applications of Bravo Ultrex 82.5WDG (22.4 oz/A) alternated with Quadris 2.08SC (6.2 fl oz/A). The chemical program was applied on a 10 day calendar program as well as when predicted by the TomCast disease forecaster. Three different prediction thresholds of 15, 20, and 25 DSVs were used to time fungicide applications. When the cumulative daily DSV values reached the determined threshold a spray would be applied. Each treatment regime was initiated at four different levels of disease pressure (0%, trace, 5%, and 10% foliar blight). The first treatments were applied on 2 July and the last application of any treatment was made on 21 September. Ten feet of each center row of the spray blocks were marked before the first application and were used for weekly disease ratings (see graphs, below). Yields were taken from the same ten feet section of row by hand harvesting the carrots and topping and weighing.
This indicates that the first treatment in carrot should be done as soon as we can find the first disease incidence in field. From now on it worked fine by the use of the TomCast model with a threshold of 20 DSV accumulated since the last spray.
Fieldclimate determines the severity of an Alternaria Infection in two different models:
Source: Jim Jasinski, TOMCAST Coordinator FOR OHIO, INDIANA, & MICHIGAN
TomCast Alternaria Model
In dependence of the climatic conditions of hours of leaf wetness and air temperature, values of severity of an Infection (from 0 – 4, see table above) are determined.
Check which sensor set is needed for monitoring this crop’s potential diseases.