The Author: 

C. H. Walkinshaw, Emeritus Plant Pathologist, U.S. Department of 

Agriculture, Forest Service, Southern Research Station, 2500 Shreveport 

Highway, Pineville, LA 7 1360. 

February 1999 

Southern Research Station 

P.O. Box 2680 

Asheville, NC 28802 
Promising Resistance to Fusiform Rust 

from Southeastern Slash Pines 

Abstract 

Two hundred twenty-four disease-free slash pines with good growth and 

form were tested for rust resistance. Trees in Alabama, Florida Louisiana, 

and Mississippi were selected. After artificial inoculations, a low 

percentage of open-pollinated progeny of 32 selected trees had galls. In 

progeny from six of those, the number of gall-resistant trees increased from 

50 to 75 percent after one round of crossing. Analysis of seedling height, 

gall length, formation of adventitious shoots, and pith necrosis suggests 

that the six families may differ in their response to infection, even though 

they form similar percentages of galls. This resistance appears to be stable 

in a variety of fungus field isolates. 

Keywords: Breeding for resistance, Cmnartium quenruumksp. Jirsiforme, 

greenhouse inoculations, Pinus elliottii Engelm. var. ellionii. 

Introduction 

Slash pine (Pinus elliottii Engelm. var. elliottii), which is 

noted for straight stems with high specific gravity and rapid 

early growth, grows well on the imperfectly drained soils of 

the middle and lower Coastal Plain (Koch 1972). Increased 

demand for wood fiber in the South between 1950 and 1970 

resulted in the expansion of pine plantations in the region. 

Slash pine was chosen for many locations because of its 

favorable early height growth, but the uniform tender shoots 

in extensive plantations provided abundant rust-susceptible 

host tissue. A full-scale fusiform rust epidemic resulted and 

eventually spread to other pine species. Infection rates in 

young stands rose to 90 percent, and mortality was often >50 

percent. 

Whereas rust-resistant slash pine trees occur throughout the 

species' range, many families with putative rust resistance 

grow more slowly than is desirable in commercial or 

national forest plantations (Snyder and others 1967, 

Wkinshaw and Bey 198 1). In this study, we detemined 

the number of trees with rust resisbnce in a population 

selected for its phenotypic growth (height md diameter at 

breast height [d.b.h.]) and form. Through control 

pollination, we sought to estimate levels of resistance. 

klaterials And Methods 

Selection of plaenoty pes-Rere are 1 77 663 hectares 

(439,000 acres) of slash pine and 12 14 f hectues (30,000 

acres) of mixed slash pine and hardwood in the national 

forests of the South. When we made selections for this 

study, roughly half of that area supported 25- to 50-year-old 

slash pines. Trees were chosen at a rate of one per every 

388 hectares of slash pine forest. Fifty trees each were 

selected in Alabama, Louisiana, and Mississippi, and 78 

were selected in Florida. These 228 trees had greater 

heights and larger diameters than neighboring trees and were 

free of fusiform rust galls. Their average volume was 26.8 

percent greater than neighboring trees, and selected trees 

were notable for their straightness and better-than-average 

crown characteristics. 

Inoculation of the progeny of selected trees-Three 

preliminary and 9 standard tests for fusiform rust resistance 

(including 1 and 2 replications, respectively) were made 

with open-pollinated seeds from 224 selections (4 did not 

produce adequate seed). Thirty-eight of the trees were 

tested more than once. Each of the nine tests consisted of 

two inoculations 1 day apart. In each run, 3 trays of 20 

seedlings were sprayed with 20,000 basidiospores per mm. 

The inoculum originated from a 150-gall mix collected from 

Alexandria, LA, Gulfport, MS, Mobile, AL, and Perry and 

Jacksonville, FX. The percentage of trees with galls was 

determined 6 months after inoculation. A tree was 

considered resistant if the mean percentage of seedlings with 

galls for the 2 inoculations was s50 percent. This level is 

considered low in tests at the Resistance Screening Center 

near Asheville, NC (Laird and Phelps 1975). All other 

aspects of the tests were modeled on procedures followed in 

earlier tests, e.g., Laird and Phelps 1975, Tainer and 

Anderson 1993, Walkinshaw and Roland 1990. 

Breeding selected trees-Six of the rust-resistant trees with 

the lowest infection rates in the greenhouse were chosen as 

breeding pruents, and, of the 228 superior phenotypes, tree 

number 2 14 was included as high1 y susceptible to fusifom 

infection. Control-pollinated trees are listed in table I (see 

page 2). 

Inoculations of control-pollinated progeny--The crosspollinated trees listed in table 1 were inoculated in j992 or 

1994. mree inoculations (replications) of 3 trays with 20 

seedlings per tray were made according to procedures 

described in ealier publications, e.g., Laird and Phelps 

1975, Tainer and Anderson 1993, Walkinshaw and Roland 
Table 1-Test years for crosses between fusiform rust-resistant siash pine 

clones' 

Females 

Males 3 10 2 14 304 21 1 33 1 224 311 

- 

R = Reciprocal cross tested. 

"Tested in either 1992 or 1994. The first digit is the State code: 1 = FL, 2 = LA, 3 =MS, 4 = AL. 

Pollinated over several years but produced inadequate seeds for the test. 

1990. A field isolate from a single gall on a commercial 

pine in Washington Parish, LA, was used to produce the 

basidiospores for the inoculations. This inoculum was 

chosen because it caused a mean infection level of 63 

percent and a range from 19 to 92 percent in 50 rust-resistant 

families on industrial and USDA Forest Service land. Bey 

and Walkinshaw (198 1) and Walkinshaw and Bey (198 1) 

provide partial lists of infections. Seedling heights (cm) and 

lengths of galls (mm) were measured 6 months after 

inoculation. Proportions of seedlings with galls (those 

containing more than two adventitious shoots at acute 

angles, and reddish-brown necrotic areas in the pith) were 

calculated 6 months after inoculation. Walkinshaw and 

others ( 1980) published a detailed description of these 

variables. 

Statistical analysis-halysis of variance tests on data 

from the control-pollinated progeny, the results of which 

were &eated as fixed effects, were performed using 

greenhouse data. Seedlings in trays were arranged in a 

randomized design within each of the three replications per 

family. Ca11 length was plotted to test for nomality. 

pollinated families. Family 2 14 was selected as the 

susceptible family in the control-pollinated study. 

In the 1992 test, 23.5 percent of seedlings from the 12 

putative resistant crosses had galls 6 months after 

inoculation (table 3). Crosses 2 1 1 x 2 11 and 3 11 x 33 1 had 

galls on only 28 and 45 seedlings per replication, 

respectively. By contrast, 58 to 60 seedlings in susceptible 

families had galls. In the control-pollinated susceptible 

seedlots, 87 to 100 percent of seedlings had galls, and in the 

Resistance Screening Center's slash pine susceptiblecomposite seedlot, 88 percent had galls. In the 1994 test, 

25.2 percent of seedlings in the 6 control-pollinated crosses 

had galls. Five crosses of slash pine 2 1 1 were included from 

another study as resistant controls and had means of 17, 18, 

28, 29, and 35 percent of seedlings with galls. 

Four traits were measured to determine any differences in 

control-pollinated seedlings that had formed galls after 6 

months (table 4, see page 4). Families were significantly 

different for the 4 traits (f = 2 1, 3 1, l l , and 8 at the 1 - 

percent level for height, length, shoots, and necrosis). The 

length of galls for family 2 11 was 26.9 mm, whereas for 

family 304, it was 53.9 mm. The majo~ty of galls on family 

3 I0 were > 50 mm. The distfi-ibution of gall lengths for the 2 

families is shown in figure la and I b. Skewing toward small from 32 of the 224 (j4 Percent) exhibited galls was common, even in the control-pollinated crosses, resisQnce, ie., z 50 percent of seedlings had galls (rabte 2, Compared to parent 33 1, twice as many galled seedlings of 

see page 3). The number of families &at were selected as paent 224 formed adventitious shoots. Progeny of parent resistant decreased when the percentage of galls for the test 2 1 I were unusual for their pith necrosis 143 percent of 

mean increased. Families selected for the control-pollinated seedlings with galls). group (table 3, see page 4) had growth similar to other open- 
Table 2-Percentage of seedlings with galls for openpoiiinated progeny of good growth parentsa 

Percentage with galls 

Family Family Test Number of families 

code means means in these testsc 

Smaller galls in inoculated progeny of parent 2 11 had less 

fungal growth in the cortex and cambium than the 

susceptible control at 2 1 and 60 days after inoculation. 

Using routine histological methods with a 12-seedling 

sample (Walkinshaw 1978, Walkinshaw and Roland 19'301, 

fungal hyphae were seen in only 2 1 percent of the specimens 

of 2 1 1 but in 100 percent of the controls 2 1 days after 

inoculation. When fungus was present, linear growth in the 

cambium at 2 1 days was 175 to 750 pm for 2 11 and 425 to 

1400 pm for the susceptible seedlot. The number of rust 

haustoria per parenchyma (cortical) cell at 40 days was 

0.073 for 2 1 1 and 0.159 for the susceptible family. There 

were 15 and 3 1 haustoria per mm' of infected cortical tissue 

for 2 1 1 and the control, respectively. 

Slash pine 2 11 was chosen as a representative to measure 

stability of the resistant families. Its open-pollinated 

progenies were inoculated with a number of rust fungus field 

isolates. The Resistance Screening Center conducted 12 

tests with different inocula on its open-pollinated progeny. 

The mean percentage of seedlings with galls across tests was 

14 percent and values ranged from 0 to 40 percent. When 9 

other field isolates of the rust fungus were used in Gulfport, 

MS, an average of 27 percent of the seedlings developed 

galls. A highly resistant Florida tree, which had a mean of 

43 percent and a range of 3 1 to 62 percent from the same 9 

inocula, was the second best tree in these tests. This 

behavior of 21 1 paralleled the 25.3-percent mean for the 

open-pollinated (table 2) and 29.5 percent for the controlpollinated families (table 3). 

Discussion 

"Mean of two replications of 60 seedlings each. 

A total of 261 seedlots were inoculated including repeats of families 

among tests. These 32 (3 repeats) resistant families are 14 percent of the 

original selections. 

' R-te test mean includes all families in an individual test. 

Selection of slash pine phenotypes with good growth and 

form that are free of fusiform rust is not difficult. Results of 

this study indicate that 14 percent of such phenotypes are 

resistant to fusiform rust, and this resistance can be 

increased to 75 percent by breeding. 

The relatively low infection of open-pollinated progeny of 

parents with good growth is encouraging because most of 

the parents were probably wind pollinated by susceptible 

trees. The number of fusiform gall-free clones may be 

increased to 75 percent when the parents are controlpollinated with resistant trees. A seed orchard of such 

highly resistant trees has been established in Gulfport, MS. 

A 0.4-hectare experimental plot of progeny from controlled 

pollinations with 2 1 1, as well as vigorous 9-2 stock, was 

planted there. The mean d.b.h. after 11 years was 14.5 cm 
Table %Percentage of slash pine seedlings with galls 6 months after 

inoculation with fusiforrn rust fungusR 

Females 

Males 3 10 2 14 304 211 33 1 224 311 

R = Reciprocal cross tested. 

Table &Mean values for traits of infected seedlings six months after 

inoculation with the rust fungusa 

1992 test 

Length Proportions Proportions with 

Familyb Height of gall with shoots pith necrosis 

crn rnrn 

" Means and proportions were determined from evaluations of 180 seedlings 

See table 3 for the percentage of seedlings with galls for these families. 

3s) is a common treatment susceptible family in both tests. 
Length Midpoint Length Midpoint 

Figure 1-Frequency of gall lengths in (a) susceptible (232 x 321) and (b) resistant families (224 x 21 1). 

(5.7 inches), compared to slash pine 9-2 progeny at 13.0 cm ~~k~~~l~d~

(5.1 inches). 

Histological results for the progeny of parent 2 11 matched 

gall lengths measured in the greenhouse. Progeny from the 

suscepti ble-control famil y contained twice the amount of 

fungal hyphae and haustoria as the progeny of parent 2 1 1. 

Slash pines with 1 to 10 mm galls have been shown to have 

no fungus 9 months after inoculations (Walkinshaw 1978). 

There is a large proportion of small galls in the frequency 

plot represented in figure 1 b. In other species of pine, small 

galls--especial1 y those close to the ground--can persist for 

over 100 years without producing major damage 

(Walkinshaw and Barnett 1995). 

The variation in rust resistance of slash pines as a function of 

the variability of field isolates remains a problem (Snow and 

others 1976). However, every effort was taken in this longterm study to expose seedlings to virulent field isolates. The 

use of many and various field isolates on the progeny of 

parent 2 1 1 had little effect on the percentage of seedlings 

with galls. Moreover, when a seedling from this highly 

resistant phenotype became infected, it demonstrated a 

mechanism for reducing host damage (Bey and Walkinshaw 

1981). 

The technical assistance of Alva Farmer, Carol H. Young, 

and James Scarborough is gratefully acknowledged. The 

help of James E. Gates and his staff over the 20-year period 

of this study is especially appreciated. 

Literature Cited 

Bey, C.E; Walkinshaw, C.H. 198 1. Stability of field resistant slash pine 

to selected isolates of fusiform rust fungus. In: Proceedings, 16th 

Southern forest tree improvement conference; 198 1 May 27-28; 

Blacksburg, VA.Virginia Polytechnic Institute and State University: 

107-1 14. 

Cornelius, J. 1994. Heritabilities and additive genetic coefficients of 

variation in forest trees. Canadian Journal of Forest Research. 

24: 372-379. 

Griggs, M.M.; Walkinshaw, C.H. 1982. Diallei analysis of genetic 

resistance to Cromrtium guerruumf: sp.fusiforme in slash pine. 

Phytopathology. 72: 8 16-8 18. 

Kinloch, B.B., JL; Walkinshaw, C.H. 1990. Resistance to fusiform rust 

in southern pines: How is it inherited? In: Rusts of pine information. 

Rep. NOR-X-3 17. Forestry Canada, Northwest Region: 219-228. 

Koch, P. 1972. Utilization of the southern pines. Agric. Handb. 420. 

Washington, DC: U.S. Department of Agriculture, Forest Service: 

27-29, 42-46. Vol. 1. 
Laird, £?I?; Plelps, W.R. 1975. A rapid method for mass screening of 

loblofly and slash pine seedlings for resistance to fusifmrn rust. Plant 

Disease Reporter. 59: 238-242. 

Snow, G.A.; Dinus, RJ.; Walkinshaw, C.H. 1976. Increase in virulence 

of Crc~nufliumfus&rme on resistant slash pine. Phytopthology. 

65: 511-513. 

Snyder, E.B.; Weley, P.C.; Wells, 0.0. 1957. Slash pine provenance 

tests. Journal of Forestry. 65: 4 14420. 

Tainer, EH.; Anderson, R.L 1993. Twenty-six new pine hosts of fusiform 

rust. Plant Disease. 77: 17-20. 

Walkinshaw, C.H. 1978. Cell necrosis and fungus content in fusiform 

rust-infected loblolly, longleaf, and slash pine seedings. 

Phytopathology. 68: 1705- 17 10. 

Walkinshaw, C.W.; Barnett, J.P. 1995. Tolerance of loblolfy pines to 

fusiforrn rust. Southern Journal of Applied Forestry. 19: 60-64. 

Walkinsttaw, C.H.; Bey, C.E 198 1. Reaction of field-resistant slash pines 

to selected isolates of Gmnurtium guenruum f: sp. hsiforme. 

Phytopathology. 7 1 : 1090- 1092. 

Walkinshaw, C.W.; Dell, T.R.; Hubbard, S.D. 1980. Predicting field 

performance of slash pine families from inoculated greenhouse 

seedlings. Res. Pap. SO- 160. New Orleans, LA: U.S. Department of 

Agriculture, Forest Service, Southern Forest Experiment Station. 6 p. 

Walkinshaw, C.H.; Roland, T.A. 1990. Incidence and histology of stemgirdling galls caused by fusiforrn rust. Phytopathology. 80: 25 1-255, 
C.H. Walkinshaw. 1999. Promising resistance to fusiform rust fkom southeastern 

slash pines. Res. Pap. SRS- 16. Asheville, NC: U.S. Department of Agriculture, 

Forest Service, Southern Research Station. 6 p. 

Two hundred twenty-four disease-free slash pines with good growth and form 

were tested for rust resistance. Trees in Alabama, Florida, Louisiana, and Mississippi were selected. After artificial inoculations, a low percentage of openpollinated progeny of 32 selected trees had galls. In progeny from six of those, the 

number of gall-resistant trees increased from 50 to 75 percent after one round of 

crossing. Analysis of seedling height, gall length, formation of adventitious 

shoots, and pith necrosis suggests that the six families may differ in their response 

to infection, even though they form similar percentages of galls. This resistance 

appears to be stable in a variety of hgus field isolates, 

Keywords: Breeding for resistance, Cronartium quercuumJ sp.&siforme, greenhouse inoculations, Pinus elliottii Engelm. var. elliottii. 
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