What pathogen causes small angular red-brown spots on trifoliate soybean leaves 2 to 3 weeks after planting?

The correct answer is Septoria brown spot, a common foliar disease in soybeans caused by the fungal pathogen Septoria glycines. This disease manifests as small, angular red-brown spots on the leaves typically 2 to 3 weeks after planting, especially under conditions of high humidity and warm temperatures. The angular nature of the lesions results from the way the fungus infects the leaves, confined by the leaf veins, which is a distinctive characteristic of this disease.

In contrast to the other options, Fusarium wilt primarily affects the vascular tissues of the plant, leading to wilting and yellowing, rather than distinct leaf spots. Anthracnose is characterized by irregularly shaped lesions and can often cause more severe impact on stems rather than specifically producing small angular spots on leaves. Root knot is a nematode issue that affects the root system leading to galls but does not directly relate to leaf spotting. Understanding these differences is crucial for correct diagnosis and effective management of soybean diseases.