DISEASE RESPONSE OF PRECOCIOUS-DWARF CASHEW (ANACARDIUM OCCIDENTALLE) CLONES TO INFECTION BY COLLETOTRICHUM GLOEOSPORIOIDES
AMQ LOPEZ1 and JA LUCAS2
1Dept. de Quimica, Universidade Fed. Alagoas, Macei 6-Alagoas, 57072-970, Brazil; 2IACR-Long Ashton Research Station, University of Bristol, Long Ashton, Bristol BS18 9AF, UK
Background and objectives
Cashew (Anacardium occidentalle) is an important agro-industrial and export crop in NE Brazil. Leaf anthracnose, inflorescence blight and rot caused by the fungus Colletotrichum gloeosporioides is one of the major production constraints in this region, and all the commercially available clones are susceptible to this disease. Prospects for conventional chemical control are limited. Therefore, understanding the process of infection, mechanisms of resistance and variation in the pathogen population can provide valuable information to optimise management of this pathogen.
Materials and methods
The response of four commercial (CCP-06, CCP-09, CCP-76 and CCP-1001) and one non-commercial (CAP-14) clone to infection by 36 C. gloeospododes isolates (LARS 905-940) from different hosts and different locations in NE Brazil was investigated under controlled environment conditions. The cytology and histochemistry of isolate/clone combinations exhibiting different disease reactions were then investigated in more detail. Both morphological and molecular approaches were employed to confirm the identity of the isolates. Finally, the effect of defence activators and potassium phosphate dibasic on susceptibility to disease was studied.
Results and conclusions
All the isolates, including those from different hosts, were able to infect and incite disease on leaves and stems of the majority of the clones, albeit to different degrees. Clone CCP-06 was the most susceptible, while clone CCP-1001 showed some level of resistance to a number of the isolates. Injury increased susceptibility of the clones to all the isolates, suggesting that resistance might be associated with structural barriers that hinder penetration. The isolates showed a high degree of morphological variability, however, the D2 sequence of their R-DNA was very similar to those reported for other C. gloeosporioides strains. The interactions between susceptible and resistant isolate-host combinations were examined in more detail. Light microscopy indicated that there was no difference between compatible (905x76, 910x76 and 910x1001) and incompatible (905x1001) interactions during the pre-penetration phase. At 36-66 h after the inoculation in a compatible interaction, a vesicle-like structure produced in the infected epidermal cell formed a large primary hyphae (LPH). At this stage, the invaded cell, although still viable, produced granulated yellow-brown pigments which were deposited with in the lumen of the cell. Further development of the LPH and the production of thin secondary hyphae (SH) was associated with necrosis and development of symptoms. In incompatible interactions, the granulated material appeared to be associated with restriction of development of the fungus (905x1001). Although this pigment was detected in both interactions, it was much less intense in compatible interactions than in incompatible interactions, particularly during the later stages of infection, when SH were ramifying through the tissue. In incompatible interactions, the first invaded cell soon became necrotic, but this did not entirely stop the advance of the pathogen. Hystochemical tests indicated that callose, polyphosphates, phenots, lignin and peroxidase were induced during both types of interaction. The main differences between the interactions were the timing and extent of the host cell's response to infection. Treatment of leaves, saplings and field-grown cashew plants with chemical such as salicylic acid, CGA-41396, CGA-245704 and potassium phosphate dibasic significantly reduced the rate of symptom development and final disease severity.