What occurs to the needles when the spores of Needlecast blacken?

When the spores of Needlecast blacken, the primary effect observed is that the needles drop off the tree. This phenomenon is typical of certain fungal diseases that affect coniferous trees, specifically those caused by pathogens like the Needlecast fungi. The blackening of spores indicates that they are reaching maturity and potentially dispersing, which is harmful to the health of the needles. As the disease progresses, it leads to the deterioration of needles, which can no longer sustain the tree and thus fall off. This shedding is a defense mechanism for the tree as it attempts to rid itself of the infected parts.

The other options suggest changes that do not directly correlate with the disease process driven by the fungal spores, such as color changes in the needles, size increases, or the development of spots, none of which capture the essential impact of Needlecast effectively. Instead, the loss of needles is a clear and critical indicator that helps in identifying the severity of the infection and the health of the tree overall.