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 germination rate is low (Döken, 1989).
For further information have a look on the homepage of CABI: //www.cabi.org/isc/datasheet/41911
Model in FieldClimate.com
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 is 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 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 periode, which lead to the formation of makrospores 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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