Inula viscosa is a perennial plant native to the Mediterranean Basin. Extracts made from the shoots of this plant exhibited a strong fungicidal activity in vitro and in vivo. TLC analyses revealed at least 7 fungicidal compounds. Most are lipophilic. When such extracts were sprayed on the leaf surface of crop plants they effectively controlled downy mildew in grape, cucumber and tobacco; late blight in potato and tomato; gray mold in cucumber and tomato; and, powdery mildew in cucurbits and cereals. Field experiments conducted with grape vine and potato resulted in effective control of Plasmopara viticola and Phytophthora infestans, respectively. The data suggest that Inula viscosa is a useful source of herbal fungicidal preparations for agricultural use.

ABSTRACT The ability of Phytophthora infestans, the causal agent of potato and tomato late blight, to produce oospores in potato tuber tissue was studied in the field and under laboratory conditions. In 1998 and 2000 field experiments, the canopy of potato cvs. Alpha and Mondial, respectively, were coinoculated with A1 + A2 sporangia of the fungus, and the infected tubers collected at harvest were examined for the presence of oospores. In 1998, only 2 of 90 infected tubers had oospores, whereas none of the 90 tubers examined in 2000 had any oospores. In the latter experiment, infected tubers kept in storage up to 12 weeks after harvest had no oospores. Artificial co-inoculations of whole tubers with A1 + A2 sporangia resulted only rarely in the formation of oospores inside the tubers. Co-inoculations of potato tuber discs taken from dormant tubers 0 to 16 weeks after harvest failed to support any oospore production, whereas discs taken from sprouting tubers of >/=18 weeks after harvest allowed oospores to form. Tuber discs showed enhanced oospore formation when treated before inoculation with either sugars, amino acids, casein hydrolysate, beta-sitosterol, or chloroethylphosphonic acid. In contrast, reducing airflow into the petri dishes where potato tuber discs were incubated reduced the number of oospores produced. The number of oospores produced in tuber tissue was lower compared with that in leaf tissue regardless of the origin of isolates used. The data show that the ability of Phytophthora infestans to produce oospores in potato tuber tissue is very limited and increases with tuber aging.

ABSTRACT Tomato fruits at the mature green stage coinoculated with A1 + A2 sporangia of Phytophthora infestans, the late blight causal fungus, showed abundant oospores in the vascular tissues, pericarp, columella, and placenta. Oospores were also formed on the surface of fruits kept in moisture-saturated atmosphere. Occasionally, oospores were enclosed between the epidermal hairs of the seed coat. In a few seeds, oospores were detected inside the embryo. The data suggest that blighted tomato fruits may carry a large number of oospores, thus making them a threatening source of blight inoculum. Such fruits may also release airborne oosporic inoculum that may introduce recombinant genotypes within a growing season. Although Phytophthora infestans is seedborne in tomato, to our knowledge, this is the first report on the occurrence of oospores in tomato seeds. Whether such tomato seeds produce blighted seedlings remains to be shown.

A leaf-disc bioassay was used to screen cucumber breeding lines, F-1 hybrids, and advanced families for resistance to downy mildew caused by Pseudoperonospora cubensis. Leaf discs (2.2 cm diam.) were taken from the third leaf from the apex of young field-grown plants, placed on 0.5% water agar and inoculated with sporangia of P. cubensis. The disease index data obtained from leaf discs were positively correlated with the disease index data collected later in the field at an advanced stage of a natural downy mildew epidemic.

ABSTRACT Two field experiments were conducted to study the effect of overhead sprinkling irrigation on oospore formation by the late blight fungus Phytophthora infestans in potato. Total rain (natural + sprinkling) accumulated in treatments of experiment 1 (winter 1997 to 1998) were 765, 287, and 219 mm and treatments of experiment 2 (winter 1999 to 2000) were 641, 193, and 129 mm. Sporangia from 11 isolates of P. infestans were combined in eight pairs, seven of A1 and A2 and one of A2 and A2 mating type, and were sprayed on field-grown potato crops (42 plants per plot at 7 m(2) each) and examined for their ability to form oospores in the host tissues. In experiment 1, oospores were recorded in a total of 132 of 1,680 leaflets (7.9%), 24 of 105 stems, and 2 of 90 tubers. In experiment 2, oospores were recorded in 40 of 519 leaflets (7.7%), but not in any of the 90 stems or the 45 tubers examined. Both the proportion of leaflets containing oospores and the number of oospores per leaflet increased with time after inoculation and were dependent on the rain regime, the position of leaves on the plant, and the isolate pair combination. In both field trials, increasing the rainfall significantly enhanced oospore production in leaves. Leaf samples collected from the soil surface had significantly more oospores than those collected from the midcanopy. Two pairs in experiment 1 were more fertile than the others, whereas the pair used in experiment 2 was the least fertile. The total number of oospores per leaflet usually ranged from 10 to 100 in experiment 1, but only from 2 to 10 in experiment 2. Maximal oospore counts in the field were 200 and 50 in experiments 1 and 2, respectively, but ranged from approximately 2,000 to 12,000 oospores per leaflet in detached leaves in the laboratory. We concluded that P. infestans can produce oospores in the foliage of field-grown potato crops, especially when kept wet by regular overhead sprinkling irrigation, but production was far below that in the laboratory.

Two inbred lines and five F-1 hybrids of cucumber resistant to downy mildew were evaluated for resistance in growth chambers and the field. Best-performing plants were selfed and S-4 plants were crossed with a high-yielding, parthenocarpic, susceptible partner. Resistance in J-13, a derivative of Wisconsin 2843, was found to be controlled by incompletely dominant gene(s). Resistance was expressed as small, chlorotic, water-soaked lesions with negligible sporulation of the fungus. No breakdown of this resistance was observed at 12 degrees C.

Dimethomorph (DMM) was effective in controlling late blight in potato and tomato caused by either metalaxyl-sensitive (MS) or metalaxyl-resistant (MR) field isolates of Phytophthora infestans and downy mildew in cucumbers and melons caused by MS or MR isolates of Pseudoperonospora cubensis. The fungicide did not affect zoospore discharge from sporangia of P. infestans but strongly inhibited zoospore encystment, cystospore germination, and mycelial growth in vitro. DMM showed translaminar activity and local systemic activity in intact plants but failed to translocate from one leaf to another in either acropetal or basipetal direction. DMM applied as a soil drench strongly protected tomatoes against late blight but failed to protect cucumbers against downy mildew. When applied in lanolin paste to the stem surface, but not when applied to the hypocotyl, it provided excellent control of both diseases in the foliage. DMM applied 1 day postinoculation partially protected potato against late blight and cucumber against downy mildew, and in both systems reduced sporulation of the pathogen. DMM diminished the sporulation of P. infestans and P. cubensis when applied to normally developed infected tissue. In P. cubensis it induced enhanced callose-encasement of haustoria. It showed a remarkable persistence on foliage subsequent to excessive washing with water as well as a high residual activity lasting for 9 days. DMM seems to be a good candidate for the control of oomycete diseases in the field especially in growing areas where phenylamide-resistant fungal populations prevail.

The effect of oxadixyl-based fungicidal mixtures on the development of late blight and the buildup of resistance ofPhytophthora infestans to oxadixyl was studied in potato crops (cv. Alpha) grown in plastic houses. Half-rates of SAN 518F (mancozeb + oxadixyl) applied weekly provided better disease control and reduced resistance buildup more effectively than the full rates of the mixture applied biweekly. Mancozeb applied weekly at full rate was less effective than SAN 518F at half-rate. The data suggest that lowering the dosage together with shortening the interval between sprays is an efficient strategy to combat mixed populations — sensitive and resistant to phenylamides — ofP. infestans when phenylamide-based fungicidal sprays are used.