Botany - B.E. Yakubenko 2017

Part Two. System of the Organic World
Chapter V. System of the Organic World
Laboratory Class Topic 10.25. Phylum Oomycota, Class Oomycetes

Object. Order Peronosporales (Peronosporales), representative — late blight, or potato fungus (Phytophthora infestans (Mont.) de Bary)

Tasks:

1. Examine and study the Features of the late blight development cycle.

2. Sketch the affected potato leaves and tubers.

3. Study and sketch the mycelium and individual Stages of the late blight development cycle.

Equipment and Materials: MBR-1 or Biolam microscopes, magnifiers, scalpels, razor blades, herbarium and fixed plant materials, late blight-infected potato leaves and tubers, slide preparations, charts, other supplies.

Macroscopic examination of potato leaves and tubers infected with late blight. Select leaves affected by late blight. Diseased plants have blackened or browned leaves. Most commonly, the tips or edges of leaf blades or their lobes turn brown. Take such a leaf and examine the affected area carefully using a magnifying Glass or under low microscopic magnification. A web-like coating with numerous conidiophores is visible on the lower side. Sketch the affected part of the leaf (Fig. 74).

Take potato tubers affected by late blight, or fixed samples, and examine them carefully. You will see gray spots on the tubers. Beneath these spots, browning is noticeable, which in some specimens covers a significant portion of the tuber. If the disease is mildly expressed and the tubers are planted, fungal sporulation may occur in the field. Sketch the late blight-infected tubers from the surface and in cross-section in your workbook.

Methods FOR PREPARING slide mounts of potato leaves and tubers. For microscopic examination of the individual stages of the parasite's development cycle, several preparations can be made. To study the harmfulness of the disease to plants and The Nature of the damage, it is advisable to make a series of thin transverse leaf sections using a sharp razor or blade. Examine them with a magnifying glass and transfer the best 2–3 sections with a fine brush into a drop of Water on a Microscope slide. Cover with a coverslip and examine the preparation under a microscope.

Microscopic examination of preparations. On the cross-section preparation of a late blight-infected leaf, you will see that the upper and lower epidermis and the mesophyll, differentiated into palisade and spongy parenchyma, stand out within the leaf blade. If the section is made at the boundary between the healthy and infected PARTS OF THE leaf, you can notice that in the infected area the Cells become brown and wrinkled, especially the Cells of the palisade parenchyma. Pay attention to the fact that some of them appear to be penetrated by tiny mycelial branches called haustoria. Through these, the fungus sucks nutrients from The Cell, after which the cells turn brown.

In certain places, you can see bundles of sporangiophores emerging through the Stomata in the lower epidermis. Examine them closely and make sure they are unicellular in Structure, lacking transverse septa. Conidiophores are continuations or outgrowths of the hyphae of the mycelium permeating the leaf mesophyll. They have a segmented apex. The branching of the conidiophores is sympodial. At their tips and branchlets, you see miniature lemon-shaped sporangia. Various stages of sporangium formation can be found on the conidiophores. After a period of rest, zoospores germinate into hyphae that penetrate into the living cells of the potato leaf mesophyll (Fig. 74).

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Figure 74. Potato late blight:

a — potato SHOOT affected by late blight; b — cross-section of an infected leaf (showing conidia on conidiophores emerging outward through the stomata); c — zoosporangium; d — release of zoospores; e — zoospores; f — zoospore germination; g — conidium; h — conidiospore germination; i — tubers affected by late blight; j, k, l — late blight mycelium in tuber cells

The second method of preparing a slide is simpler. Place a drop of water on a clean microscope slide and add a portion of the white fungal coating from the lower side of the affected leaf part. Use a dissecting needle for this, preferably with a flattened tip. Moisten the tip of the needle with water, pick up a small amount of the fungal coating from the potato leaf, and transfer it into the drop of water on the microscope slide. Cover with a coverslip. A preparation from a potato tuber can be made as follows. Cut the tuber at the site of the most pronounced symptom manifestation. Make a series of thin sections with a sharp razor. Using a magnifying glass, select the best sections that encompass the tuber right from The surface of the affected area. Place them in a drop of water on a microscope slide, cover with a coverslip, and examine under a microscope.

On the preparation of the potato tuber section, you will see areas of affected storage tissue. The cells have changed little in shape and size, but their contents have become different from those of living cells. The affected cells are filled not only with starch grains, but also with hyphae of the late blight mycelium. Sketch them and label the indicated parts for the various preparations.

Conclusion. The mycelium of oomycetes (late blight) is coenocytic (non-septate), unicellular. Reproduction and dissemination of the fungus occur via zoospores. The mycelium destroys the leaf mesophyll, reducing crop yields. By infecting potato tubers, late blight causes potato rot.

Self-Control Questions

1. What kind of mycelium do Peronosporalean Fungi and Phytophthora have?

2. Name and explain the modes of reproduction in oomycetes.

3. What environmental factors contribute to the spread of diseases caused by oomycetes?

4. Characterize the development cycle of late blight and its individual stages.