Y. Levy

Joint action of fungicides in Mixtures: Theory and practice

Fungicide mixtures aimed at the control of multiple target organisms have been used since the middle of this century. In the 1970s synergistic mixtures of site-specific (high resistance-risk) fungicides with multi-site (low resistance-risk) fungicides became available with the aim to provide adequate disease control while maximizing evolutionary problems for the pathogen. In the 1980s there were introduced mixtures of negatively correlated fungicides, in which each ingredient is capable of controlling pathogen genotypes resistant to the other ingredient. Such fungicides are currently deployed in the form of homogeneous mixtures (tank mixes, prepacked mixtures). A new strategy is under development in which different fungicides are applied to different adjacent plants. Diversification of cultivars and fungicides in a crop may minimize the buildup of resistance and maximize disease control.

Epidemiology of Northern leaf blight on sweet corn

The epidemiology of northern leaf blight of corn, caused byExserohilum turcicum (Pass.) Leonard and Suggs, is reviewed. The minimal dew period required for infection is temperature-dependent. At 25°C, 1 h of dew is sufficient to cause infection and at this temperature the minimal dew period for sporulation is 14 h. Under natural conditions when one dew night is not long enough for conidia to develop, the dew period on the following night enables the completion of conidial formation. The amount of conidia formed is dependent on temperature, light, plant age, leaf position and plant susceptibility. Both qualitative and quantitative types of resistance were identified in several hybrids. Subsequently, there developed additional biotypes ofE. turcicum which are aggressive to plants containing qualitative monogenic resistance. Within the same physiological race, a significant variation in aggressiveness between isolates from various locations is observed.The pathogen overwinters as mycelia and conidia in infected leaves, husks and other plant parts, or onSorghum halepense L. Reduction in yield due to northern leaf blight is associated with the level of resistance of the host plant, with disease severity, plant age during infection, and position of infected leaves.

The overwintering ofExserohilum turcicum in Israel

Exserohilum turcicum (Pass.) Leonard and Suggs, the causal agent of northern leaf blight of corn, overwinters onSorghum halepense L. plants and on corn debris (dead leaves). Spqrulating lesions ofE. turcicum were observed on sorghum plants in the winter (February). Spores from these lesions were pathogenic to susceptible sweet corn plants cv. ‘Jubilee’. Infected sporulating leaves of corn were stored for 1 year at 20°C (40-60% relative humidity), at 5°C (60% relative humidity), or buried 5 cm underground. During the storage period, 32% and 22% of the spores formed chlamydospores, at 20° and 5°C, respectively. Leaves buried 5 cm underground were totally decomposed after 6 months. After 4 months, 25% of the spores in the buried leaves had formed chlamydospores. Spores with chlamydospores were pathogenic to corn plants. The viability of spores without chlamydospores stored at 20°, 5°C or buried underground was 0, 60 and 0%, respectively. In a parallel experiment infected leaves were stored under similar conditions and allowed to sporulate. No sporulation occurred on infected leaves buried in soil. Spores produced on infected leaves stored at 20° and 5°C were highly pathogenic to corn plants. In leaves treated with 0.1N glucose, chlamy dospore formation was significantly inhibited.

Physiological races ofExserohilum turcicum in Israel

A set of differentials of corn plants(Zea mays L.) containing Ht1, Ht2, Ht3 or HtN genes was used to identify races ofExserohilum turcicum in Israel. Plants were inoculated with 14 isolates ofE. turcicum collected from various regions in Israel (from Ayyelet HaShahar in the north to Sa’ad in the south). Differentials containing Ht1, Ht2, Ht3 or HtN genes were resistant to the 14 isolates tested, whereas the inbred lines without Ht genes were highly sensitive. Resistance was characterized by the formation of non-sporulating chlorotic lesions. When plants containing Ht1, Ht2 or Ht3 genes were inoculated with relatively high inoculum concentrations (over 50 conidia/drop), chlorotic lesions were associated with necrosis in the center of the lesions. Sporulation of the fungus in the necrotic parts of the lesions was significantly less than on plants without Ht genes. No necrosis was observed in plants with the HtN gene. Our results indicate that the physiological race ofE. turcicum in Israel is race 1.

Yield loss of sweet corn cultivars in response to defoliation and to infection byexserohilum turcicum

The effect of northern leaf blight (NLB) or leaf defoliation on the yield of nine sweet corn cultivars was studied in field experiments. Plants were defoliated or infected in the bottom, middle or top third of the leaf canopy. The effect of defoliation or infection on yield was significantly higher for plants treated 55 days after planting than for those treated 70 days after planting. Cultivar response differed significantly for both treatments, the most sensitive cultivar being ‘Jubilee’ and the least sensitive being Ssupersweet 7900. In all cases, the highest yield loss was recorded when the middle third of the leaf canopy was treated. The relative importance of the top third of the leaf canopy increased if the treatment was done 3 weeks before harvesting. This study emphasized the importance of leaf position, cultivar, and plant growth stage for evaluating losses caused by NLB, and may explain the failure of various models to describe yield loss due to NLB.

Mating types ofExserohilum turcicum in Israel

Fifty-one single conidial isolates ofExserohilum turcicum were collected from seven locations in Israel and three in the USA and tested for mating type and aggressiveness. Sixteen isolates mated were mating type a; 29 were mating type A; and six failed to mate with either the A or the a mating type. Isolates differed significantly in their ability to cause lesions of different sizes, and in their ability to produce conidia on the susceptible corn hybrid Jubilee. They also differed in their rate of growth on lactose casein agar. No significant difference in lesion size or conidial production was noted between mating types. Conidial production and radial growth were significantly lower for isolates with no mating capacity as compared with conidial production of defined mating types. Pseudothecia were produced after 14–16 and 21 days when mating occurred onSorghum halepense straw or barley straw, respectively. The average number of pseudothecia produced on 10 cm2 of straw was 17 and 8 forS. halepense and barley, respectively. Pseudothecia were not produced on corn straw.

Synergistic interactions between phenylamide fungicides and respiration inhibitors in controlling phenylamide-resistant populations ofPhytophthora infestans on potato tuber disks

The efficacy of mixtures of respiration inhibitors and phenylamide fungicides (oxadixyl and metalaxyl) in controlling late blight was investigated using potato tuber disks. Results showed that uncoupling agents(e.g. DNP), inhibitors of ATP formation(e.g. oligomycin), ionophores(e.g, valinomycin) and inhibitors that block specific carriers in the electron transport chain(e.g. rotenone, antimycin A) increased significantly the control efficacy of both metalaxyl and oxadixyl towards populations ofPhytophthora infestons when applied to tuber slices pretreated with sublethal doses of both fungicides. When applied alone at the same doses, respiration inhibitors did not inhibit the development of the late blight fungus. Increased control efficacy of mixtures was not noted against phenylamide-sensitive populations of the fungus.

Abstracts of papers presented at the 4th Meeting on Pheromone Research in Israel Abstracts of papers presented at the 9th Congress of the Phytopathological Society of Israel

S OF PAPERS PRESENTED AT THE 4TH MEETING ON PHEROMONE

Involvement of a phytotoxic peptide in the development of the Northern leaf blight of corn

Non pathogenic isolates ofExserohilum turcicum successfully infect corn plants in the presence of syntheticE. turcicum toxin during inoculation. The toxin significantly increased the number of appressoria and the ramification of germinating conidia both on host leaves and on artificial media. These findings indicate that this toxin plays an important role in infection of Northern leaf blight.