Plant Protection Institute, HAS, Budapest, Hungary

Background and objectives
The infection of sunflower by Plasmopara halstedii [Farl.] Berl. et de Toni is usually born either from infected seeds or from oospores persisting in the soil for years but often becomes evident at the four leaf stage of the host. The syndromatic picture of the sunflower downy mildew (SDM) is very complex. The typical symptoms (root rot, stunting, leaf chlorosis, sporulation) are not always observable for each infected individual, the disease sometimes manifests only in slight growth retardation of the plant. The rotation of host dependent and host independent periods in vegetative generations of ontogenesis is characteristic to this pathogen, and its relationship with host plant changes from commensalism of the penetrating hyphae to obligatory endobiotrophic parasitism of the systemically invading thallus. Selection of sunflower for resistance to downy mildew has not given resounding success, and only the acylanilide fungicides provide sufficient protection. The developing resistance to these fungicides however necessitates the search for new compounds with new modes of action.

Results and conclusions
In a multiyear study, using test methods developed earlier [1,2], we have been examining the effect of over 300 natural and synthetic substances on SDM. We have found that P. halstedii exhibited highly selective developmental stage dependent response to chemicals. In addition, some compounds (sabadilla alkaloids, digitogenin, GABA, tridemorph), having no activity on the sporulation, were able to localize the pathogen on the infected area. We have concluded that parameters for evaluation, are highly sensitive to environmental influences. Reliable information about the effect of chemical treatments can be obtained by evaluating a) presence or absence of the pathogen on the area of penetration, b) its apical growth through the next internode, and c) its penetration from stem into the true leaves. Most substances affected just one developmental form of P. halstedii (zoospores or penetrating hyphae mainly) and only a few of them inhibited the pathogen in more than two developmental stages (ceveratrum alkaloids, Hg-compounds). Apparently, the host plant protects the endobiotrophic pathogen from the toxic effect of exogenic compounds. In general the pathogen in its host dependent stages (parasiting hyphae) was less sensitive than in host independent ones (asexual spores). The zoosporangia were the most tolerant event and the sensitivity of other developmental forms increased in the following order: zoosporangia themselves < systemic invasion of the thallus < sporulation < zoosporogenesis < zoospores < oospores. Osmotic shock renders zoosporangia sensitive to cationic detergents indicating the special character of the contact between the plasmalemma and the cell wall of sporangia in determination their tolerance to chemicals.
As P. halstedii can infect the sunflower through all parts, and from zoo- and oospores, the most sensitive developmental forms are not easily accessible in the field, only the eradication of parasiting thallus and the inhibition of oospore formation seem to be successful control measures.

1. Oros G & Virányi F, 1987. Ann. appl. Biol. 110, 53-63.
2. Virányi F & Oros G, 1991. Mycol. Res. 95, 199-205.