Istituto di Patologia Vegetale dell'UniversitÓ degli Studi di Milano, Via Celoria 2, I-20133 Milano, Italy

Background and objectives
Onion downy mildew (Peronospora destructor (Berk.) Casp. = P. schleidenii Unger) is heavily influenced by weather conditions with special regard to epidemiology and symptom expression [1, 2]. The disease was reported in north-western Italy as a harmful one since 1892 by Briosi and Cavara [3]. Nevertheless, in spite of the economic importance of this crop, no studies on the biology and epidemiology of onion downy mildew in Italy have been published, to our knowledge. The aim of this note is to describe early symptom variability observed in onions grown for bulbs in order to allow early detection of downy mildew infections and promote timely and suitable control [4].

Materials and methods
Epidemiology and symptomatology of onion downy mildew were studied near Pavia, in north-western Italy, where onions are usually sown directly in field at the end of February or in early March and harvested in July-August. Periodic observations, sometimes daily, were carried out during growing seasons from 1993-97 in several farms, also considering symptoms in onion field trials performed to compare 31 cultivars, different in bulb colour and time of ripening.

Results and conclusions
Onion downy mildew was never noticed during the years from 1993 to 1995. In 1996 the disease was observed at first on cv. Borrettana and then affected other cultivars towards their harvest time. Symptoms were observed firstly at the beginning of June, in small areas of a field of cv. Borrettana, typically densely sown, then (10 July) they appeared also in cv. Density fields. P. destructor revealed itself by sporulation on leaf surfaces without any evident damage of tissues. Dark grey moulds, formed by sporangiophores emerging from stomata, covered portions, often wide, of external leaves especially at their top or in the middle part. 2 weeks later, leaves showed chlorotic areas and tip necrosis. External leaves were more damaged than inner ones and bent at the upper part. Finally, necrotic area size increased and whole leaves turned light brown and dried.

Some typical symptoms, such as chlorotic spots with pathogen sporulations on green tissues surrounding them, appeared along the edges of affected field areas. Chlorotic spots turned brown, sometimes purple. Around the middle of July, symptomatic and asymptomatic plants showed small, light brown necrotic spots, without any pathogen sporulation. In 1997 almost all the onion cultivation underwent heavy losses due to the disease. The first secondary infection symptoms became evident on 7th June, following a windy and scarcely rainy May, as little, light brown necrotic spots localized in the upper half of old leaves. P. destructor sporulation was not detected immediately on injured leaves, while it was abundantly found 21 days after. Following this detection most of onion leaves rapidly dried, causing important yield losses.

Independent of weather conditions, it turned out that early varieties are more susceptible to the disease than late ones. Meteorological data showed that disease establishment was strictly connected to solar radiation intensity, the increase of which seems to give rise to increased disease presence. It is important to note that solar radiation and global environmental conditions heavily influence plant physiology and the pathogen life cycle. Dew formation plays a special role in onion downy mildew development: plants growing on headlands, where dew dries faster than elsewhere, showed very low disease severity. The particular symptomatologies connected with the first infection of onion downy mildew in 1996 and 1997 emphasized the difficulties in early diagnosis. Therefore these symptomatological aspects could be assumed to be important factors in the start of disease control.

1. Palti J, 1989. Phytoparasitica 17, 31-48.
2. Viranyi F, 1981. In Spencer DM, ed., Downy mildew. Academic Press, London, pp. 461-472.
3. Briosi G, Cavara F, 1892. I funghi parassiti delle piante coltivate od utili 7, 151.
4. Quaroni S, Saracchi M, Cavagna B, 1998. Informatore Fitopatologico (in press).