Main symptoms

Downy mildew can attack all the aerial organs of the tomato . It is characterized by the development of spots patches initially wet , or even , on the leaflets (Figure 1). These attacks locally give the affected tissues a tint pale green to green-brown . Large portions of the leaf blade are eventually affected and soon turn brown and necrose (Figures 2 and 3). These spots are frequently surrounded by a livid, ill-defined margin of tissue on which sometimes forms, on the underside of the blade, a discreet and fleeting white down consisting of the sporocystophores and sporocysts of P. infestans . When conditions are particularly favorable, the progression of symptoms on the leaflets is rapid. Leaves, twigs and even plants eventually die off and dry out completely (Figure 4).

Of canker lesions more or less extended, brown , irregularly-shaped, are visible on the rods (Figure 5) and petioles . They often encircle them more or less. Similar brownings can be observed on floral bouquets; they are at the origin of the fall of many flowers.

The fruits reached at an early stage have brown mottling very characteristics (Figure 6-8). They are often dented . In this case, the extension of the mottles is rather slow and their margin irregular. If the attacks occur later, the mottled areas are more homogeneous and often distributed in concentric scalloped circles. They can lead to confusion with those caused by other Phytophthora , in particular P. nicotianae .

The same whitish fuzz as seen on the leaflets is occasionally visible on the surface of the fruit . The latter remain relatively firm regardless of the precocity of the attack. In some cases, secondary microorganisms can invade the damaged tissues and lead to various rather soft rots. Note that some seeds from diseased fruits would be the source of contamination of future seedlings.

Let us remember that the diseased plants are distributed in foci in the plots ; these can increase rapidly if the weather conditions are good.


1. Analysis of the mitochondrial and nuclear loci of P. infestans allows us to support the hypothesis that this chromist would originate from South America. An ancestral population would have diverged into several lineages in the South American Andes in association with various Solanum. Two of these divergent lines would have given rise to the existing P. infestans haplotypes capable of infecting potatoes, tomatoes and some wild species of Solanum.

2. There are about 800 saprophitic or parasitic species of Oomycetes. The latter have long been classified in the Phycomycetes or "lower fungi" (Eumycetes). This classification was questioned a few years ago because their ultrastructure, biochemistry and molecular sequences indicated that they belonged to the Chromists, especially including algae (green and brown), diatoms A controversy seems to remain around their classification. Many authors would associate them with the reign of the Chromista and the phylum Heterokonta, others would place them in the reign of the Straminipila.

3. Several other strains have been isolated in the United States, especially in recent decades:
- the US-6 clonal line, infecting both tomatoes and potatoes (in 1979);
- two clonal lines identified in 1992 in the United States and Canada, US-7 and US-8, both of the A2 mating type, resistant to metalaxyl and also pathogenic on the two Solanaceae;
- a recent isolate US-17, sexual type A1, showing high aggressiveness on tomato comparable to that in US-8, and resistant to metalaxyl.