Diaporthe helianthi (stem canker of sunflower).

No effect of D. helianthi on wild Helianthus spp. in North America has been documented. Though the fungus could be said to have invaded Europe, its spread has probably been mediated by human activity rather than by natural means. In any case, sunflower is an annual exotic crop in Europe, planted in monoculture, and the disease has not affected the extent of sunflower cultivation. So D. helianthi cannot be said to be invasive.

Quantitative PCR Assays Developed for Diaporthe helianthi and Diaporthe gulyae for Phomopsis Stem Canker Diagnosis and Germplasm Screening in Sunflower (Helianthus annuus)

Phomopsis stem canker of sunflower is caused by two fungal pathogens, Diaporthe helianthi and Diaporthe gulyae, in the United States. In this study, two quantitative PCR (qPCR) assays were developed to detect and quantify D. helianthi and D. gulyae in sunflower. The two assays differentiated the two fungi from each other, other species of the genus Diaporthe, and pathogens, and they have high efficiency (>90%). The qPCR assays detected the two pathogens on plant samples exhibiting Phomopsis stem canker symptoms sampled from commercial sunflower fields in Minnesota, Nebraska, North Dakota, and South Dakota. Furthermore, the assays were used to screen cultivated sunflower accessions for resistance to D. helianthi and D. gulyae. The disease severity index (DSI) of the accessions significantly correlated (P < 0.0001) with the amount of pathogen DNA from the qPCR assays. The qPCR assays identified PI664232 and PI561918 to be significantly less susceptible (P ≤ 0.05) to D. helianthi and D. gulyae, respectively, when compared with the susceptible check cultivar HA 288, and this was in agreement with the DSI. These results suggest that the qPCR assays for D. helianthi and D. gulyae can be used as a reliable tool to diagnose Phomopsis stem canker and screen sunflower germplasm for disease resistance.

Comparison of Greenhouse-Based Inoculation Methods to Study Aggressiveness of Diaporthe helianthi Isolates Causing Phomopsis Stem Canker of Sunflower (Helianthus annuus)

Phomopsis stem canker is an economically important disease of sunflower (Helianthus annuus), and Diaporthe helianthi is one of the primary causal agents of the disease in the United States. The objective of this study was to evaluate inoculation methods of D. helianthi isolates on sunflower in the greenhouse. Four isolates of D. helianthi were selected to test the effectiveness of four inoculation methods using mycelial plugs as the inoculum, including stem wound, wound inoculation, petiole wound, and straw test. Infection was established by the D. helianthi isolates at 14 days after inoculation for all inoculation methods used. However, recovery of the pathogen from the inoculated plants differed significantly (P < 0.0001) among inoculation methods. Given higher mean recovery of D. helianthi isolates from the inoculated plants and the size of the lesions caused by the pathogen, the stem-wound inoculation method was found to be the most user friendly of the four inoculation methods.

Identification of Sunflower (Helianthus annuus) Accessions Resistant to Diaporthe helianthi and Diaporthe gulyae

Phomopsis stem canker is an important disease on sunflower (Helianthus annuus) in Minnesota, North Dakota, and South Dakota. To date, Diaporthe helianthi and D. gulyae have been reported as the common and primary causal agents in this region. The objectives of this study were to compare aggressiveness of the two species and evaluate the USDA cultivated sunflower germplasm collection for resistance to the two pathogens. Five D. helianthi isolates and five D. gulyae isolates were compared for their aggressiveness on the USDA susceptible confection inbred ‘HA 288’ using the stem-wound inoculation method. The results of this study demonstrated that D. helianthi and D. gulyae isolates did not vary significantly (P > 0.05) in their aggressiveness. Using a representative isolate of D. helianthi and D. gulyae, 49 accessions were screened for Phomopsis stem canker using the stem-wound method in the greenhouse. Among the 49 accessions, 13 and 4 accessions were less susceptible to D. helianthi and D. gulyae, respectively, compared with HA 288. Only one accession (PI 552939) was observed to be significantly less susceptible to both D. helianthi and D. gulyae when compared with HA 288.

The response of sunflower hybrids to different plant densities on a chernozem soil

In our experiment the reaction of six sunflower hybrids of different genotypes (NK Oktava, ES Biba, ES Diagora, ES Ballistic, EGH 8925, PR 64 H 42) towards plant density has been investigated by different fungicide treatments in the crop-year of 2011 on a chernozem soil. In the crop-year of 2011 sunflower populations were infected by a significant Diaporthe helianthi disease. The extent of this infection was significantly enhanced by the increment of plant density. However, regarding the average of the hybrids and plant densities the two times executed fungicide treatment has decreased the infection rate by 22%. The most susceptible hybrid was the ES Biba. Contrarily, the hybrid ECH8925 proved to be the most resistant hybrid towards thisdisease according to our results. According to the results of the Pearson’s correlation analysis it has been revealed that stalk breakage and Diaporthe infection stand in a very close (r=0.782**) and middle close (r=0.523**) correlation resp. with plant density. The relationship between fungucude treatments and stalk breakage, just as Diaporthe infection showed to be middle and close respectively. Our results demonstrate the role of stalk and plate diseases (among them Diaporthe) in causing stalk breakage, for we have found a close positive correlation between stalk breakage and Diaporthe infection (r=0.624**) in our analysis.From the aspect of yield amount the optimal plant density varied between 45 000 and 55 000 plants per hectare. Fungucude treatments enabled not only the use of higher plants densities, but they had a yield increasing effect as well. In the crop-year of 2011 the highest yield (4 559 kg ha-1) on a chernozem soil has been measured in case of the hybrid ECH8925.