Impact of Rag1 aphid resistant soybeans on Binodoxys communis (Hymenoptera: Braconidae), a parasitoid of soybean aphid (Hemiptera: Aphididae).

Multiple strategies are being developed for pest management of the soybean aphid, Aphis glycines Matsumura; however, there has been little published research thus far to determine how such strategies may influence each other, thereby complicating their potential effectiveness. A susceptible soybean (Glycine max L.) variety without the Rag1 gene and a near isogenic resistant soybean variety with the Rag1 gene were evaluated in the laboratory for their effects on the fitness of the soybean aphid parasitoid, Binodoxys communis (Gahan). The presence or absence of the Rag1 gene was verified by quantifying soybean aphid growth. To test for fitness effects, parasitoids were allowed to attack soybean aphids on either a susceptible or resistant plant for 24 h and then aphids were kept on the same plant throughout parasitoid development. Parasitoid fitness was measured by mummy and adult parasitoid production, adult parasitoid emergence, development time, and adult size. Parasitoids that attacked soybean aphids on susceptible plants produced more mummies, more adult parasitoids, and had a higher emergence rate compared with those on resistant plants. Adult parasitoids that emerged from resistant plants took 1 d longer and were smaller compared with those from susceptible plants. This study suggests that biological control by B. communis may be compromised when host plant resistance is widely used for pest management of soybean aphids.

Reproduction of Soybean Cyst Nematode on Dry Bean Cultivars Over Multiple Generations.

Phaseolus vulgaris is a host of soybean cyst nematode (SCN; Heterodera glycines), a pathogen recently introduced into the major dry bean production area of North Dakota and northern Minnesota. The nematode reproduces less on most bean classes compared with soybean but can reduce plant growth and seed yield. An important question is the following: will SCN adapt to dry bean and, over time, increase in ability to reproduce on roots? To answer this question, the following experiments were conducted with cultivars from three bean classes. The cultivars 'Premiere' and 'Cirrus' (navy), 'Buster' and 'Othello' (pinto), and 'Eclipse' and 'Jaguar' (black) were grown in "Cone-tainers" in sand in plastic pots immersed in a water bath at 27°C in the greenhouse. Seedlings were inoculated with 2,000 eggs per plant of SCN HG 0 and cysts were harvested and counted after 40 days. The eggs were immediately extracted from those cysts and seedlings were inoculated again and grown for 40 days using the same methods. Soybean 'Lee 74' was used as a control. A female index (number of cysts produced on the test plant divided by the number of cysts produced on Lee 74) was calculated for each bean cultivar after each period of 40 days. This procedure was repeated until eight generations of eggs were completed and then the experiment was repeated. There was no significant (P ≤ 0.05) change over time in the female index on the six bean cultivars. Therefore, there was no evidence that SCN HG 0 was increasing reproduction on dry bean cultivars during two 11-month periods of continual reproduction of HG 0 on roots.

Reaction of Soybean Cultivars to Isolates of Fusarium solani from the Red River Valley.

Five isolates of Fusarium solani, originally isolated from diseased soybean roots in the Red River Valley (RRV) of Minnesota and North Dakota, were evaluated for their ability to cause symptoms on 10 genetically diverse soybean cultivars. Taproots of 2-week-old plants were inoculated with F. solani-infested oat kernels, and 3 and 10 weeks later, plants were evaluated for root rot and foliar symptoms. At 3 weeks after inoculation, taproots of all cultivars had extensive reddish brown to black lesions; root rot severity (1-6 scale) ranged from 4.8 to 5.1, and 3.5% of the plants had died. Foliar symptoms were not observed. At 10 weeks after inoculation, all cultivars showed extensive decay of taproots and >50% of lateral roots were necrotic; root rot severity (1-4 scale) ranged from 2.7 to 3.7, and 42.5% of the plants had died. Foliar symptoms were first observed between the R-1 to R-6 growth stages (about 5 weeks after inoculation) on the lower leaves and consisted of chlorosis at the margins that progressed inward. Veins initially were green, but leaves eventually became chlorotic, then necrotic, and fell with petioles still attached to the stem. In some cases, all of the foliage died. There was no significant (P = 0.05) isolate × cultivar interaction for root rot at 3 or 10 weeks after inoculation or for severity of foliar symptoms. Thirty-three cultivars commonly grown in southern Minnesota and the RRV were evaluated for reaction to one isolate of F. solani. Root rot severity ranged from 4.2 to 5.7 (1-6 scale) and 3.5 to 4.0 (1-4 scale), at 3 and 9 weeks after inoculation, respectively, and >50% of the plants died by 9 weeks after inoculation. Severity of foliar symptoms was low. These results indicate that isolates of F. solani from the RRV cause root rot and foliar symptoms on soybean and that cultivars grown in the region lack resistance to this pathogen. Foliar symptoms were not identical to those associated with sudden death syndrome.