Proc. of
Evaluation of Field
Trees for Resistance to Phytophthora cinnamomi by Means of the Detached Root Technique
Marius De
V. van der Merwe, Erna M.C. Maas, and Johannes M. Kotze
Department of Microbiology and Plant
Pathology,
Abstract. Avocado trees selected by SAAGA for exceptional growth under apparent root rot pressure were evaluated for resistance by means of the detached root technique. Mycelium fragments of Phytophthora cinnamomi were used as an inoculum source instead of zoospores. As means of comparison root segments from root-stocks such as G755, Duke 7 and Edranol with known responses towards Phytophthora cinnamomi were used. Nine of the 34 trees evaluated were as resistant as G 755 and six were significantly more resistant than Duke 7. Most of the trees, (i.e. 25) were significantly more resistant than Edranol.
The search for resistance to Phytophthora cinnamomi Rands, the causal organism of avocado root rot, was initiated by Dr. G.A. Zentmyer in 1952 (Zentmyer, 1952). According to him, collections have been made in 18 countries and include 15 species of Persea and species of other genera in the Lauraceae. Resistance to P. cinnamomi in these collections was tested in a nutrient solution test (Zentmyer and Mircetich, 1965), in pots and beds of P. cinnamomi infested soil and ultimately in the field (Zentmyer, 1952).
In South Africa, no indigenous Persea species occur and the search for resistance is thus restricted to orchard trees showing exceptional signs of vigor under apparent root rot pressure. These trees have been termed "escape" trees.
Obtaining clonal material from these trees for use in resistance tests is a long and tedious procedure. The aim of this study was therefore to evaluate the detached root technique described by Botha, Wehner and Kotze (1989) as a rapid means to assay field trees for resistance to root rot.
Materials and Methods
Roots were obtained from trees selected by Mr. C. Partridge and Mr. D. Westcott of SAAGA. Locality and tree designation are shown in Table 1.
Root tips excised from the different trees were placed separately in plastic containers, filled with moist, sterile vermiculite to prevent dehydration and contamination. The containers were kept in cool boxes, transported to the laboratory and tested within 24 h.
As a control and means of comparison, root tips from two-year-old P. americana cultivar Edranol (susceptible) (Snyman et a/., 1984) and vegetatively propagated (Frolich and Platt, 1971) P. americana selection Duke 7 (moderately tolerant) (Coffey, 1987) as well as P. schiedeana Nees selection G755 (tolerant) (Coffey, 1987) were used.
The detached root technique used to test for resistance in avocado rootstocks as described by Dolan and Coffey (1986) and modified by Botha et al, (1989) was used. However, inoculum of P. cinnamomi consisted of 10 μL of mycelium suspension. For the mycelium inoculum, 20 5 mm2 potato dextrose agar discs (PDA) previously colonized by P. cinnamomi were inoculated into 100 mL pea broth prepared as described by Chen and Zentmyer (1970). After shake-incubation at 25C for four days the fungal growth was homogenized for 30 s with an ultra turrax to produce a mycelial suspension.
The excised root tips (ca. 40 mm in length) from each of the different trees, as well as those from the control trees, were placed perpendicularly onto two parallel glass rods in petri dishes containing 15 mL water agar in each as described by Botha et al. (1989). Each root tip was inoculated at the region of elongation with 10 μL of the mycelium homogenate and incubated in the dark at 25C.
Resistance was determined by aseptically cutting the root tips in 4 mm segments after surface disinfecting for 5 s in 70% ethanol. The root segments were then plated out sequentially on PARPH-medium. After incubation at 25C for three days, the segments from which P. cinnamomi developed were counted and multiplied by four to obtain the total length of root colonization.
To evaluate whether time after field removal of roots affected expression of resistance, the potted control trees were initially taken to the field and the roots were excised at the same time as those of the field trees. Half of the roots from the control trees were immediately taken to a nearby laboratory and tested as described above. The other half of the roots was kept in the same manner as the root tips of the field trees, until the tests were performed 24 h after detachment.
For all further tests on the field trees,
the root tips of the potted control trees remained at the
Results and Discussion
Time after root detachment (within a 24 h period) did not significantly affect expression of resistance (Table 2). Linear colonization of the excised roots of field trees from each locality as well as the controls are given in Tables 3-6.
Nine of the 34 trees tested showed a higher degree of resistance than G755, although this difference was not significant. Six and 25 trees were found to be significantly more resistant than Duke 7 and Edranol, respectively. Three of the 34 trees tested were significantly more susceptible than Edranol.
According to Zentmyer
and Mircetich (1965) preliminary tests for resistance
of rooted cuttings are conducted in a nutrient solution inoculated with P. cinnamomi However, due to the time required to obtain
rooted cuttings, an alternative method for evaluating resistance of field trees
was investigated. It was found that the detached root technique described by Botha et al. (1989) could readily be used when
controls such as G755, Duke 7 and Edranol were
included in each evaluation. Thus results of this study showed some field trees
to be as resistant as the highly acclaimed G755. No previous reports on the
"resistant status" of existing avocado trees in
We wish to thank Mr. C. Partridge, Mr. D. Westcott and Mr. N. Claassens of SAAGA, for the demarcating of escape trees and assistance.
Literature
Cited
Botha, T., F.C. Wehner, and J.M. Kotze. 1989. An evaluation of in vitro screening techniques for determining tolerance of avocado rootstocks to Phytophthora cinnamomi. S. A. Avocado Growers' Assn. Yrbk. 12:60-63.
Chen, D.W. and G.A. Zentmyer. 1970. Production of sporangia by Phytophthora cinnamomi in axcenic culture. Mycologia 62:379-402.
Coffey, M.C. 1987. A look at current avocado
rootstocks.
Dolan, T.E. and M.D. Coffey. 1986. Laboratory screening technique for assessing resistance of four avocado rootstocks to Phytophthora cinnamomi. Plant Disease 790:115-118.
Frolich, E.F. and R.G. Platt. 1971. Use of the etiolation
technique in rooting avocado cuttings.
Snyman, A.J., C.P. Snyman, and J.M. Kotzé. 1984. Pathogenicity of avocado root rot fungi to Edranol seedlings and Duke 7 rooted cuttings. S. A. Avocado Growers' Assn. Yrbk. 7:80-81.
Weste,
G. 1974. Phytophthora cinnamomi
- The cause of severe disease in certain nature communities in
Zentmyer,
G.A. 1952. Collecting avocados in
Zentmyer, G.A. 1984. Avocado Diseases. Trop. Pest Mgmt. 30:388-400.
Zentmyer, G.A. and S.M. Mircetich. 1965. Testing for resistance of avocado to Phytophthora in nutrient solution. Phytopathology 55:487-489.
|
Table 1. Locality and designation of field
trees evaluated for resistance to P. cinnamomi by
means of the detached root technique. |
|
|
Locality |
Tree designation |
|
Agatha |
A2, B1, B3, B4, B12, B13, B14 |
|
Burgershall |
1D1, 1D2, 1D3, 1D4, 1D5, 1D6, 1D7, 1D8,
1D9, 1D12, 1D13, 1D14 |
|
Levubu |
2B1, 2B2, 2B3 |
|
Nelspruit |
1Q1, 1Q2, 1Q6, 1Q7, 1Q8, 1 Q9 |
|
Tzaneen |
Z4 |
|
|
1AV, 2AV |
|
|
C1, C2, 1L1 |
|
Table
2. Linear colonization of potted avocado roots by P. cinnamomi
at different time intervals after detachment. |
||
|
Rootstock |
Root
colonization |
|
|
Evaluation
0 h after detachment |
Evaluation
24 h after detachment |
|
|
Edranol Duke 7 G755 |
20.45 a z 6.00 b 1.67b |
18.00 a 3.40 b 2.00 b |
|
z Values not followed by the same letter differ
significantly according to |
||
|
Table 3. Linear
colonization of excised root tips of
field trees by P. cinnamomi after 48 hours; Agatha and Tzaneen sites. |
|
|
Rootstock |
Linear
colonization (mm) of roots |
|
Edranol |
18.0az |
|
B3 |
12.45
ab |
|
Z4 |
12.13ab |
|
B13 |
11.00
abc |
|
B4 |
9.55
bed |
|
A2 |
7.27
bed |
|
B1 |
6.42
bed |
|
B14 |
4.73
bed |
|
B12 |
4.44
bed |
|
Duke
7 |
3.40
cd |
|
G755 |
2.00
d |
|
z Values not followed by the same letter differ
significantly according to |
|
|
Table 4. Linear
colonization of excised root tips of field trees by P. cinnamomi
after 48 hours; Nelspruit and |
|
|
Rootstock |
Linear colonization (mm) of roots |
|
Edranol 1Q2 Duke 7 1Q7 1Q6 1Q9 C2 G755 C1 1L1 1Q8 1Q1 |
26.08 a z 16.71 be 14. 27 be 13. 83 be 11. 73 bed 7.72 bed 5.85 cde 3.81 cde 2.00 de 2.00 de 0.75 e 0.50 e |
|
z Values not followed by the same letter differ
significantly according to |
|
|
Table 5. Linear
colonization of excised root tips of field trees by P. cinnamomi
after 48 hours; Levubu and |
|
|
Rootstock |
Linear colonization (mm) of roots |
|
Duke 7 2B3 1AV 2B2 Edranol 2AV G755 2B1 |
15.54az 14. 84 a 14.00 a 13.46 ab 13.33ab 12.00ab 10.00 ab 1.54b |
|
z Values not followed by the same letter differ
significantly according to |
|
|
Table 6. Linear
colonization of excised root tips of
field trees by P. cinnamomi after 48 hours; Burgershall
site. |
|
|
Rootstock |
Linear
colonization (mm) of roots |
|
Edranol |
27.5
az |
|
Duke
7 |
21.0ab |
|
1D8 |
20.8
ab |
|
1D12 |
19.3
ab |
|
1D4 |
18.0
ab |
|
1D6 |
15.2
b |
|
1D9 |
14.7
b |
|
1D13 |
12.5
be |
|
1D1 |
12.4
be |
|
1D3 |
11.7
be |
|
G755 |
11.5
be |
|
1D5 |
1 1 .4 be |
|
1D2 |
11.3
be |
|
1D14 |
10.6
be |
|
1D7 |
1.5
c |
|
z Values not followed by the same letter differ
significantly according to |
|