[pj46] TITLE Effects of soil-borne Rhizoctonia solani Kuhn on yield and quality of ten potato *potato* cultivars
INTRO Rhizoctonia solani Kuhn is a serious pathogen of potato ( Solanum tuberosum L. ) , causing *potato* stem canker and stolon pruning. Infections can occur from both tuber- and soil-borne inoculum. Scholte ( 1987 ) showed that he infection through soil-bourn inoculum depended on the frequency with which potatoes had been grown; preceding *potato* crops other than potato had little effect, a finding confirmed by Specht & Leach ( 1987 ).
The effects of *potato* stem canker and stolon pruning caused by R. solani, on growth, yield or quality have been investigated by Cother & Cullis ( 1985 ) , Roth ( 1985 ) , and Hide et al. ( 1985 ) . There was *potato* stem canker and stolon pruning in their experiments, but whereas Hide et al. artificially infected their *potato* seeds *potato* tubers, in the other two studies infection could have been caused by soil- and/or tuber-borne inoculum.
This paper describes experiments designed to elucidate the contribution of R. solani to the yield depression of potato in short rotations. The effect of soil-borne inoculum on *potato* stem and stolon attack in relation to yield and quality of the potato were investigated. An experiment with long and short rotations supplied excellent inoculum, but had only one *potato* cultivar. The effects of *potato* cultivar were studied in two experiments with severe levels of soil infestation.
METHOD In 1983, 1984 and 1985, 18 *potato* plants of the cv. Element were harvested individually from each of 32 potato *potato* plots 74 and 117 days after planting in a rotation experiment described by Scholte ( 1987: Experiment 1 ) . The levels of R. solani infection, numbers of *potato* stem and *potato* tubers, and fresh yield, dry matter yield and dry matter content of tops and *potato* tubers were measured for each of 3456 *potato* plants.
In 1986, cv. Mirka was included in the experiment by dividing each *potato* plot into two: 83 days after planting, 18 *potato* plants of each *potato* cultivar were harvested per *potato* plot and infection by R. solani assessed.
This experiment was laid out in 1984 on a *potato* field of light sandy soil, containing 2.2% organic matter and with a pH-KCl of 5.2, where potatoes ( 1982 ) and maize ( 1983 ) had previously been grown.
In November 1983, 30 000 l ha-1 cattle slurry was applied to the *potato* field and then ploughed early in March 1984. Before planting, N, P and K fertilizers at 201, 22 and 83 kg ha-1, respectively, were incorporated with a cultivator.
Two very early maturing *potato* cultivars, Eersteling and Ostara, and two very late maturing *potato* cultivars, Multa and Alpha, were compared at two *potato* stem densities, 12 and 24 *potato* stems per m2, achieved by planting one or two *potato* seed *potato* tubers per hill.
The experiment was a randomized complete block design with four replicates. The *potato* plots were 9.0 x 4.5 = 40.5 m2, of which 7.5 x 3.3 = 24.75 m2 was harvested. *potato* Seed *potato* tubers were planted on 8 April at 75 x 30 cm. Weeds, aphids and late blight were controlled.
Two weeks before the expected maturing date, 110 *potato* plants per *potato* plot were individually harvested. Rhizoctonia infection, *potato* stem number, and fresh and dry matter yield of *potato* tubers were recorded for each of 3520 *potato* plants.
This experiment was laid out in 1985 on light sandy soil ( 2.2% organic matter, pH-KCl 5.2 ) in a *potato* field where maize ( 1983 ) and potatoes ( 1984 ) had been grown.
To encourage R. solani, the *potato* field was treated with aldicarb ( Temik 10G, Union Carbide Benelux, Maarsen, 10% a.i., 30 kg ha-1 broadcast ) one day before planting.
The early maturing *potato* cultivars Eersteling, Saskia and Vindika and the late maturing *potato* cultivars Alpha, Amigo and Baraka were compared for their sensitivity to R. solani.
Experimental design, *potato* plot size, *potato* plant spacing and other cultural practices were as for Experiment 2. The planting date was 10 April. Harvesting and recording of 2640 *potato* plants was also done as in Experiment 2 but *potato* tuber quality was additionally assessed.
In all experiments R. solani attack was classified as
Yields in the group with class 1 attack were found not to have suffered ( Experiment 1 ) and infections in this group could not irrefutably be attributed to R. solani, therefore classes 0 and 1 were combined in Experiment 2 and 3. As there were few *potato* plants of class 4 in Experiments 1 and 2, class 4 was combined with class 3.
Before planting, *potato* seed *potato* tubers ( 32 - 35 mm ) visibly free from black scurf were immersed in a solution of validamycine ( Solacol, AAgrunol, Haren, 30 g I-1 a.i., 3 % solution ) to control superficial R. solani mycelium.
For Experiments 1 and 2, *potato* seed *potato* tubers were not pre-sprouted. For Experiment 3, *potato* tubers were pre-sprouted for two weeks in light and short, dark sprouts developed on the *potato* tubers. Planting was done by hand.
RESULTS These are the results, we are not using them.
DISCUSSION Because the *potato* seed *potato* tubers planted in the experiments were free from black scurf and had been disinfected, only soil-borne inoculum of R. solani could be held responsible for infections.
Severe attacks reduced the yield and quality of *potato* tubers but a slight or moderate attack had little or no effect. The fresh *potato* tuber yields of severely and very severely attacked *potato* plants were reduced by ca. 16 and 21%, respectively, and dry matter *potato* tuber yields were reduced by ca. 20 and 29%, respectively. Marketable yield was reduced even more because the proportion of *potato* tubers > 35 mm decreased and the proportion of misshapen *potato* tubers greatly increased.
Roth ( 1985 ) and Cother & Cullis ( 1985 ) also used an individual *potato* plant sampling procedure to study the effects of R. solani infections on yield. However, because they did not disinfect their *potato* seed *potato* tubers, their R. solani *potato* stem and stolon infections could have been caused by soil-bourne and/or tuber-bourne inoculum. In severely attacked *potato* plants, Roth found fresh *potato* tuber yields reduced by 13 -24 %, depending on *potato* cultivar. Cother & Cullis found that when all stolons of a *potato* plant were pruned, fresh *potato* tuber yield was reduced by 24 %.
Hide et al. ( 1985b ) inoculated *potato* seed *potato* tubers in the ridge at planting and found yields reduced by 4 % compared to their controls. Chand & Logan ( 1982 ) found, averaged over ten *potato* cultivars, a fresh yield reduction of 18% in *potato* plots artificially inoculated with R. solani compared with uninoculated *potato* plots.
Other workers have applied specific fungicides to the soil to study the effects of soil-bourne R.solani on yield. Cother ( 1983 ) and Davis ( 1978 ) used pesticides based on quinozene ( PCNB ), but this is not a good standard because, at the recommended dosages, it is phytotoxic to potato, lowering yields ( Van Emden, 1958 ) . In two experiments on sandy soils, Moulder & Roosjen ( 1982 ) planted disinfected *potato* seed *potato* tubers in *potato* plots untreated and treated with tolclofos-methyl, furmecyclox and pencycuran. At the recommended dosages, dry matter yields of *potato* tubers were 4 - 13 % higher on treated than on untreated *potato* plots.
The success of soil-applied fungicides depends on the inoculum level of R. solani. In *potato* plots on sandy soil continuously cropped with potato Scholte ( 1987 ) found 30% severely attacked *potato* plants, averaged over four years. The yield reduction of such a *potato* crop can be calculated with formulae derived from the results of the experiments discussed in this report. The relationships between the percentage of severe ( S ) and very severe ( V ) attacks of *potato* stem stolons caused by R. solani and the related total fresh ( F ) and dry matter ( D ) *potato* tuber yield can be expressed as :
For *potato* crops with 30% severely attacked *potato* plants, the fresh and dry matter *potato* tuber yields are decreased by about 5 % and 6 %, respectively. As these values are close to the limit for significance at P < 0.05, it is difficult to prove that the beneficial effects of soil applied fungicides are statistically significant.
The negative effect of an R. solani infection on *potato* tuber yield tends to decrease towards the end of the growing season: for example 36 % lower yield on day 74 and 20 % on day 117 for severely attacked *potato* plants ( Table 1 ) . A similar effect was found by Griffith ( 1984 ) and Hide et al. ( 1985b ) .
Young sprouts are very susceptible to R. solani infection and early infection can prune off a *potato* stem as occurs especially with tuber-borne rather than soil-borne inoculum ( Frank, 1987; Van Emden et al., 1966 ) . This phenomenon may explain why the effects of attack on the number of main *potato* stems were small in Experiment 1 and 2.
Van Emden ( 1965 ) concluded that when sprouts were exposed to light after emergence they became resistant to R. solani but this does not always seem to be true. Scholte ( 1987 ) , Roth ( 1985 ) and Hide et al. ( 1985a ) found a marked increase in *potato* stem infection after emergence. Susceptibility seems to decrease gradually with increasing age of *potato* stems, probably due to a change in the structure and composition of the outer-most cell layers. Similarly, sprouts formed in the light are not attacked by R. solani but when *potato* seed *potato* tubers that have been pre-sprouted under light are planted deeply, the part of the sprout which forms between the original sprout tip and the soil surface is again susceptible to R. solani ( Roosjen & Mulder, 1982 ) .
Cother & Cullis ( 1985 ) found no significant relationship between *potato* stem canker and *potato* tuber yield an concluded that only stolon pruning results in lower yields. This seems debatable; the effect of stolon attack may be more important for yield than *potato* stem attack, but *potato* stem attack also affects growth of the *potato* plant. A severe *potato* stem attack on a young *potato* plant results in diminished growth ( Table 1, day 74 ) . Clearly, the mean weight of the *potato* stems is lower for severely attacked than for healthy plants; this is true in young *potato* plants even for those moderately attacked. However, a severe *potato* stem attack is often associated with severe stolon attack, a relationship also found by Weinhold et al. ( 1982 ) . However, late stolon attack may occur without a *potato* stem infection.
Initially, *potato* stem canker and stolon pruning have a large depressive effect on total *potato* plant weight. However, *potato* tuber yield is reduced more than haulm yield, and *potato* tuber number is also reduced. Stolon infection delays *potato* tuber initiation and , as a result, *potato* tuber yield is slower to develop. *potato* Stem canker and stolon pruning change the distribution pattern of dry matter within the *potato* plant. Translocation of carbohydrates from the haulm to underground *potato* plant parts is hampered, dry matter accumulates in the haulm so that in attacked *potato* plants the haulm dry matter content is higher and that in the *potato* tubers is lower compared with healthy *potato* plants. These trends are maintained later in the growing season but the initial differences in haulm weight disappear. Hide et al. ( 1985b ) found a lower weight for main *potato* stems, but this was compensated by a higher weight for lateral *potato* stems. In severely attacked *potato* plants *potato* tuber number per *potato* plant was initially lower, but later in the growing season it was higher than in *potato* plants that had not been attacked. In very severely attacked *potato* plants, the Ôlittle potatoÕ *potato* plants, only a few *potato* tubers remained on the underground *potato* stem bases; they usually grew into large *potato* tubers, whereas near the soil surface many small *potato* tubers developed.
*potato* Plants severely and very severely attacked by R. solani appear to be continuously in a state of secondary growth, resulting in deformed *potato* tubers ( elongated and knobby *potato* tubers with protruding eyes ) . The second-growth phenomena are probably the result of fluctuating hormone levels in the *potato* plant. *potato* Tubers situated near the soil surface, sometimes partly exposed, are also exposed to higher temperatures and this also promotes second growth. the dormancy of these *potato* tubers is low ( Gadewar et al., 1980 ) .
The effects of an R. solani attack on *potato* tuber yield and quality depend on the *potato* cultivar and perhaps, on circumstances. For example in Experiment 2, the two late maturing *potato* cultivars were attacked more severely than the two early maturing *potato* cultivars. The *potato* seed *potato* tubers had been planted directly from cold store and were not pre-sprouted so that the late maturing *potato* cultivars emerged much later than the early maturing ones. The latter therefore ÔescapedÕ R. solani attack.
Of the two early maturing *potato* cultivars, Eersteling was more seriously ( though not significantly so ) attacked than Ostara, whereas of the late maturing *potato* cultivars, Multa was much more severely attacked than Alpha. In Experiment 3, *potato* seed *potato* tubers were pre-sprouted, differences in date of emergence were small, R. solani attack was serious, and the differences in attack between *potato* cultivars were statistics significant, but were not related to the earliness of the *potato* cultivar. Differences were also found in the number of very severely attacked ( Ôlittle potatoÕ ) *potato* plants. Cvs Alpha and Baraka had the same number of *potato* plants the were at least severely attacked, but the proportion of Ôlittle potatoÕ *potato* plants was much higher for cv. Alpha than for cv. Baraka. Cv. Eersteling had very many Ôlittle potatoÕ *potato* plants.
Experiment 1 also revealed that *potato* cultivars differ in susceptibility. The level of attack was twice as high in cv. Mirka as in cv. Element. Difference between *potato* cultivars in susceptibility to R. solani were also found by Assenov ( 1986 ) , Bogucka ( 1983 ), Plamadeala ( 1978 ) , Frank et al. ( 1976 ) , Dowley ( 1972 ) and Hofferbert & Orth ( 1951 ) .
One conclusion from Experiment 2 and 3 is that if *potato* cultivars are to be compared for their resistance to R. solani, precautions should be taken, such as pre-sprouting the *potato* seed *potato* tubers, to ensure that the time between planting and emergence is about the same for all *potato* cultivars.
In Experiments 2 and 3 there was no evidence that *potato* cultivars differ in sensitivity to R. solani; at the same level of attack there were no significant differences in yield reductions.
Severe and very severe attacks of *potato* stems and stolons by soil-borne R. solani decreases fresh yield, dry matter yield and dry matter content of *potato* tubers, increases the proportion of deformed and small *potato* tubers, but have a negligible effect on haulm yield and *potato* stem number.
Slight and moderate attacks have very small or no effects on yield and quality.
The relative fresh ( F ) or dry matter ( D ) yield of *potato* tubers can be estimated from the percentage of severely ( S ) and very severely ( V ) attacked *potato* plants by the formulae.
Potato *potato* cultivars differ in susceptibility to *potato* stem and stolon infection by R. solani, but there is no evidence that they differ in sensitivity.