Weather Variables Associated with Infection of Tomato Fruit by Colletotrichum coccodes

Onion Thrips (Thysanoptera: Thripidae) Host Plant Preference and Performance are Mediated by a Facultative Plant Pathogen of Onion

Insect vector and phytopathogen interactions are mediated by host plants. Insects interact with pathogens directly or indirectly and they may prefer host plants based on infection status. Performance on infected hosts varies depending on the type of pathogen involved. Species specific studies of economically important insects and phytopathogens are needed to understand how these interactions impact crop yields. Onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), is an economically devastating insect pest of onions (Allium cepa L., Asparagales: Amaryllidaceae) worldwide and it co-occurs simultaneously with many different pathogens. Colletotrichum coccodes (Wallr) (Glomerellales: Glomerellaceae) is a generalist fungal pathogen that attacks onion foliage, causing tan lesions and decreasing yield. Onion thrips and C. coccodes represent two important pests of onions, but the relationship between onion thrips and C. coccodes infected onions has not been studied, and it is unclear if onion thrips contribute to the spread of C. coccodes in onion fields. A four-choice test with control, artificially injured, artificially injured + symptomatic, and inoculated-symptomatic onion suggests that onion thrips distinguish between hosts based on health status. Furthermore, a two-choice test with control, inoculated-asymptomatic, and inoculated-symptomatic onion pairings revealed that onion thrips distinguish between hosts based on infection status and prefer inoculated-symptomatic hosts. In a no-choice test, onion thrips numbers increased on inoculated-symptomatic plants compared to control or inoculated-asymptomatic plants. Overall, we found that onion thrips preferred and performed best on C. coccodes infected plants.

Fertile sporophore production of Typhula phacorrhiza in the field is related to temperatures near freezing

Two field tests and one lab test were conducted to examine the environmental factors affecting sporophore production in Typhula phacorrhiza and to compare these results with those documented for T. ishikariensis and T. incarnata. In the 2001 lab test where lighting, soil moisture, and soil-sand media were tested in 50 mL screw-cap tubes incubated at 4 degrees C, the limiting factor for Typhula sporophore production was found to be moisture. In the fall 2001 field test, 100 sclerotia of six isolates from three Typhula spp. were placed into pots filled with a sand and soil mixture. The pots were monitored weekly, and maximum sporophore production for all six isolates and for watered and unwatered pots was observed at 11 weeks, which was soon after mean daily temperatures fell below 0 degrees C. In the second field test in fall 2003, five isolates of the three species were tested with similar procedures, but peak sporophore production was observed after 6 weeks, and again only after mean daily temperatures fell below 0 degrees C. In the field, sporophore production of T. phacorrhiza seems to require the same environmental cues as those of T. ishikariensis or T. incarnata, namely high moisture and temperatures near freezing.

Appressorium Formation and Tomato Fruit Infection by Colletotrichum coccodes

Tomato fruit grown for commercial processing are harvested when the majority of the fruit are at the full, red-ripe stage of development. At this physiological stage, marketable yields often are reduced significantly by Colletotrichum coccodes. Appressorium formation and the infection of tomato fruit by C. coccodes were investigated in controlled-environment experiments. Conidia of C. coccodes were subjected to five temperature treatments (10 to 34°C with 6°C increments), and eight incubation periods (3 to 24 h with 3-h increments). The highest proportion of conidia that formed appressoria occurred at 16 and 22°C. Appressoria were formed within as few as 6 h of incubation at 16, 22, and 28°C. In contrast, incubation periods of at least 15 and 18 h were required for appressoria to form at 34 and 10°C, respectively. Appressorium formation was significantly reduced by 0.1 to 0.2 ppm of the fungicide chlorothalonil, and no appressoria formed at concentrations >0.4 ppm. When tomato fruit were inoculated with C. coccodes at three inoculum densities (2 × 10⁵, 6 × 10⁵, and 10 × 10⁵ conidia/ml) and incubated in dew chambers for 8, 16, and 24 h at 5°C increments from 15 to 35°C, there was no significant interaction among inoculum density, dew period, and temperature. In general, across all inoculum densities and dew periods, anthracnose severity levels were greater for each 5°C increase in temperature from 15°C until its maximum level was observed at 30°C. However, when the fruit were exposed to 35°C, disease development was minimal. At temperatures from 15 to 30°C anthracnose severity increased proportionally as dew-period duration and inoculum density increased.

Influence of Inoculum Density, Wetness Duration, Plant Age, Inoculation Method, and Cultivar Resistance on Infection of Pepper Plants by Colletotrichum coccodes

Effects of inoculum density, wetness duration, plant age, inoculation method, and cultivar resistance on the development of pepper anthracnose caused by Colletotrichum coccodes were evaluated under controlled environmental conditions. As inoculum density of C. coccodes increased from 10³ to 10⁶ conidia/ml, symptoms of anthracnose developed. Disease severity also increased with increasing time of wetness duration from 0 to 60 h. Wetness duration above 48 h and a high inoculum density at 10⁶ conidia/ml caused severe defoliation and blight symptoms in pepper seedlings. Inoculum at 10⁴ conidia/ml and 24 to 36 h of continuous wetness duration was enough to form irregular or circular brown lesions on pepper leaves. Age-related resistance was expressed as pepper plants matured. To evaluate resistance, the foliar-inoculation method was more reliable than the soil-drench method, which did not induce symptoms on older plants. The responses of 35 pepper cultivars or accessions tested to C. coccodes were quantitative rather than qualitative. No hypersensitive symptoms were observed in any of the cultivars inoculated with any of the two isolates. No significant differences in susceptibility to anthracnose were found among cultivars from Korea, the United States, India, and Thailand or accessions tested, irrespective of genetic or country origin of cultivars. The Korean cultivar Kwangbok was relatively less susceptible to anthracnose, compared with the other cultivars tested.