Figure 1. A) Late blight lesions from which Phytophthora infestans is sporulating. The white “fuzz” is sporulation (sporangia on sporangiophores). Sporulation only occurs after several hours of leaf wetness or very high relative humidity. B) Sporangia on sporangiophores as viewed through a compound microscope. C) A sporangium that has detached from a sporangiophores. Sporangia detach from sporangia relatively easily in changing relative humidity. D) A germinating sporangium releasing a zoospore. E) Potato leaflets with several very young late blight lesions. These lesions resulted from inoculations 3-4 days previously.
Figure 2. Late blight infection on tomato leaflets. A) Infection by a strain of P. infestans that is not specialized to tomatoes results in smaller necrotic lesions. B) Infection by a strain of P. infestans specialized to tomatoes results in massive colonization and sporulation before necrosis becomes evident.
Figure 3. Potato tuber infected by P. infestans. The corky, granular necrosis in the tuber tissue is characteristic of tuber blight. This tuber was harvested just minutes before the photo was taken, so infection clearly was initiated before harvest – perhaps 1-2 weeks before this picture was taken.
Figure 4. Infected tip of a potato stem, with necrosis and sporulation on the stem, petioles and leaves.
Figure 5. Tomato fruits, leaves and stems infected with P. infestans. Sporulation is evident on some of the leaflet lesions. (courtesy of T. A. Zitter).
Figure 6. Infected green tomato fruits from which P. infestans is sporulating. (courtesy of T. A. Zitter).
Figure 7. Close-up of infected potato leaflet illustrated in Figure 1.
Figure 8. Closeup of micrograph of sporangia on sporangiophores
from Figure 1.
Figure 9. Infected potato tubers harvested from plants with infected foliage.
Figure 10. Potao leaflets with very young late blight lesions.
Figure 11. Potato tuber infected with P. infestans – obtained from plants with infected foliage.
Figure 12. Potato tuber infected with P. infestans. The corky, granular symptoms are characteristics of tuber blight.
Late blight on potato leaves. P. infestans is sporulating from these lesions. Sporulation is typically visible early in the morning after a very wet night.