Science practice 94

PASSAGE II

Asian soybean rust (ASR) is a disease caused by the fungus Phakospora pachyrhizi. ASR spreads by windborne spores that infect soybean leaves. As rust lesions mature, they produce thousands of additional spores. Over time, large spore loads build up within fields and across large geographical areas. In 2004, this disease was detected in nine states in the American southwest, and by 2005 it had invaded several other states. ASR can drastically reduce crop yields in areas where it commonly occurs, so monitoring and application of preventive measures such as fungicide will likely be necessary.

Certain fungicides have been tested for their effectiveness against ASR. These fungicides are listed in Table 1. The simplest classification of fungicides divides them into three categories: contact, locally systemic, and systemic. Properties of these fungicide categories are given in Table 2.

ASR infections generally begin in the lower leaf canopy where humidity is higher and leaves stay wet for longer periods. For this reason, the lower soybean leaf canopy is the primary spray target. Both upper and lower leaf surfaces must be sprayed. Coverage as dense as 400 spray droplets per square inch is considered ideal.

The different properties of fungicide types have important implications for spray application. Contact and locally systemic fungicides require better spray coverage than systemic fungicides. Contact fungicides, because they do not penetrate the plant tissue, are more easily washed off the leaf by rain. This results in a shorter residual control period and more frequent re-application of the fungicide.

Tests have shown that fungicide effectiveness varies based on the soybean growth stage at which the fungicide is applied. Figure 1 identifies some of the different stages of soybean growth. Soybean leaves can be infected at any time with ASR. However, research has shown that the most critical time to protect soybean plants with fungicides is from the R1 through R3 growth stages. Fungicide applications should not be initiated after the R4 growth stage (seed development and mature plant).

Figure 1