In modern palaeontology, the consequences of the meteor strike in the Yucatan 1.3 million years ago are now accepted as one of the dominant hypotheses as to how the dinosaurs went extinct. Beyond the initial destruction and shock wave, the dust thrown into the air significantly lowered the amount of sunlight which could reach the earth's surface. The result was a long-term shift toward a dimmed, coolish climate. This would have been beneficial to any fungi, and indeed the fossil record for this period of time gives evidence of a significant spurt in fungal growth and diversity.
Many forms of fungi occupy pathogenic niches, preying on or otherwise taking advantage of the dominant species of their time. Since fungi often rely on the dominant species to keep providing it with a suitable reproductive environment, fungal pathology usually arises as a side effect of fungal parasitism rather than as the direct effect of the fungus. For example, ergot only has pathological effects when it is ingested. However, ingestion of grain which has been infected with ergot makes it impossible for the ergot to reproduce, and so infected grain shows clear visual markers of the infestation, making it less likely that the grain will be ingested and cause ergotism.
Thus, in a stable climate and environment, most species of fungi achieve dynamic equilibrium with the dominant species upon which it relies. Even where the relationship is pathological, it won't normally be fatal to the large majority of the dominant species. Neither species would benefit from that.
However, where the climate changes too rapidly for an ecology to compensate, fungal infestation may suddenly increase in frequency, or may turn out to have much more severe consequences than usual. Using the ergot example, a sudden change in climate which too strongly favours ergot infestation will make it much more likely that ergot-infested grain will be consumed by human beings. At the extreme, the entire crop may be lost to ergot, while at the same time the ergotism epidemic decimates the human population. This cycle of ergotism epidemic became the norm during the mid to late Middle Ages in Germany and France, a pattern which was interrupted only by the Great Famine (1315-22), followed in short order by a general drying of the climate, the Little Ice Age, and the Black Death. At that time, it would have taken very little to tip the human species in Europe over the edge of catastrophe into extinction.
What if a similar kind of thing happened to the herbivore dinosaur's food supply?
The coprolites, or fossilized excrement, of Isisaurus colberti show that its main diet consisted of a fairly wide variety of tree leaves, most of which also coexisted with various fungi including Erysiphe and Uncinula, which cause powdery mildew and black spot, and Colletotrichum, which causes leaf spot and red rot disease. The diet further suggests that the original climate of Isisaurus was tropical to subtropical. Even a slight shift of climate toward cooler and damper would likely have encouraged fungal growth. We cannot know for certain, but it is likely that one or more of these fungi reduced the edibility of its host leaves, or actively killed off the tree, or had a pathogenic effect on Isisaurus. The more widespread the fungus, the greater this effect would be.
What affects the herbivores of an ecology will eventually filter up to affect even the top of the food chain. The die-off of the dinosaurs seems to have been a protracted event, taking as long as 300,000 years. As the herbivores slowly went into decline, were the last of the carnivorous dinosaurs starved into extinction?
Probably we will never know exactly what caused the disappearance of the dinosaurs, but it is quite likely that explosive fungal growth played a significant role.
Sharma N et al. (2005) Fungi in dinosaurian (Isisaurus) coprolites from the Lameta Formation (Maastrichtian) and its reflection on food habit and environment. Micropaleontology 51(1), 73-82.